restart:

#
# Statement not terminated in the following - fixed it
#
  B[0]:= 1.7e8:  C[0]:=0:  P[0]:=0.1: E[0]:=2:
#
# Why does this (commented) Digits command exist?
#
#Digits:=10:

#
# OP never uses 'gamma' in this worksheet - why the
# hell unprotect an inbuilt variable for no purpose???
#
unprotect('gamma'):


 A__h := 10:
 beta__1 := 0.34e-1:
 psi__o := 0.25e-1:
 mu := 0.4e-3:
 theta := .7902:
 alpha := .11:
 k := 0.136e-3:
 z := 0.5e-1:
 delta := .7:
 eta := .134:
 xi := .12:
 beta := 0.1e-1:
 Q1 := 1:
 Q2 := 1:
 h := .1:
tau:=1.0;
  kappa:=2.0;
#
# The variables 'tau' and 'kappa' are used in the following,
# but neither of these is defined. Gave them a "random"
# values here just to assist debug
#
  
 

#Step 1
n:=100:
#
# Fixed the names of all of the following for some kind
# of consistency
#
lambda1[n] := 0: lambda2[n] := 0: lambda3[n] := 0: lambda4[n] := 0: u0[0] := 0:

for i from 0 to n-1 do

     B[i+1] := h*(theta*E[i]+A__h+B[i])/(1+ psi__o*h*P[i])/(P[i]+xi)+mu+u0[i]+beta*C[i];
#
# Following statement is a recursive assignment.
# It was a recursive assignment the last time the
# OP posted some code, and it is still a recursive
# assignment now!!!!
#
# Let's see if I can make this explanation simple enough
# for the brain-dead.
#
# C[i+1] is undefined. Therefore, assigning C[i+1] in
# terms of C[i+1] is doomed to failure.
#
# Changing C[i+1] to C[i] on the rhs is *may* be incorrect
# but at least the statement can now be evaluated!!
#
#     C[i+1] := h*B[i+1](beta*C[i]-psi[o]*P[i])/(P[i]+xi)-(alpha+mu+k)*C[i+1];
#
# I'm also assumin that psi[o] ought to be psi__o, so
# I made this change
#
     C[i+1] := h*B[i+1](beta*C[i]-psi__o*P[i])/(P[i]+xi)-(alpha+mu+k)*C[i];
     P[i+1] := h*z*C[i+1]/(1+delta+eta);
     E[i+1] := h*(alpha*C[i+1]+B[i+1]*u0[i]+E[i])/(1+theta+mu);

#
# The following statement has one more oopening parenthesis (ie '(')
# than closing parenthesis - again basic syntax
#
# I randomly added a closing parenthesis, just to make it syntactically
# correct, but the odds that I got this closing parenthesis in the
# desired location are almost nil!!!!!
#
#   lambda1[n-i-1] := -h*(Q1+(beta*C[i]+(psi__o*P[i]/(P[i]+xi)))*lambda2[n-i]-lambda1[n-i]-mu+u0[i]/(1+beta*C        [i]+P[i]/(P[i]+xi));
#
  lambda1[n-i-1] := -h*(Q1+(beta*C[i]+(psi__o*P[i]/(P[i]+xi)))*lambda2[n-i]-lambda1[n-i]-mu+u0[i])/(1+beta*C[i]+P[i]/(P[i]+xi));
#
# The variable 'kappa' in the following is undefined
#
  lambda2[n-i-1] := -h*(-beta*B[i+1]*lambda1[n-i-1]+Q2-alpha-mu-kappa+z*lambda3[n-i]+alpha*lambda4[n-i]-lambda2[n-i])/(-beta*B[i+1]+1);

  lambda3[n-i-1] := h*((psi__o*B[i+1]*P[i+1]/(P[i+1]+xi)^2+theta)*lambda1[n-i-1]-lambda2[n-i-1]*psi__o*B[i+1]*P[i+1]/(P[i+1]+xi)^2-lambda3[n-i])/(1+delta+eta);

  lambda4[n-i-1] := h*lambda4[n-i]/(1-theta-mu);
#
# The variable 'tau' was undefined, so (obviously) the
# following cannot be fully evaluated
#

     R1:= (lambda1[n-i-1]-lambda4[n-i-1])*B[i+1]/tau;
#
# Since R1 has not been fully evaluated, u0[i+1] cannot
# be fully evaluated either - and so the chaos continues!
#
    
      u0[i+1] := min(1, max(R1, 0));    
    end do:

#
# These two command are assigned to nothing. They are
# are never used for anything - why do they exist??
#
seq(i,i=0..30):
seq(B[i],i=0..30):

with(plots):

with(DEtools):

#
# The quantity 'h', was earlier defined to be 0.1
# Thus h(t)=0.1, h(0)=0.1 and so on. Probably not
# what the OP desired. Even funnier is when the
# OP uses initial conditions such as h(0)=400,
# which of course will evaluate to 0.1=400. This
# is a whole new level of misunderstanding
#
# Obviously this mean that the definition of these
# ODEs is completely incorrect - so it is a complete\
# waste of time trying to solve them!
#

DE1 := diff(a(t),t) = A__h-beta__1*a(t)*g(t)-psi__o*a(t)*v(t)/(v(t)+xi)-mu*a(t)+theta*m(t);
DE2 := diff(g(t),t) = beta__1*a(t)*g(t)-psi__o*a(t)*v(t)/(v(t)+xi)-(alpha+mu+k)*g(t);
DE3 := diff(v(t),t) = z*g(t)-(delta+eta)*v(t);
DE4 := diff(m(t),t) = alpha*g(t)-(theta+mu)*m(t);

SIRsys := [DE1, DE2, DE3, DE4];
#
# The commands DE1plot(), DE2plot(), DE3plot() DE4plot()
# do not exist in Maple so the following will never
# evaluate. OP (probably??) means DEplot(). However with
# the incorrect use of the name 'h' described above,
# even a syntactically correct command will not
# produce anything meaningful. Correct command with
# crap arguments will still fail!!
#

aplot := DEplot(SIRsys, [a(t), g(t), v(t), m(t)], t = 0 .. 100, a = 0.167e9 .. 0.17e9, [[a(0) = 0.17e9, g(0) = 0, v(0) = 4, m(0) = 2]], scene = [t, a(t)], thickness = 2, linecolor = red, stepsize = .1);

gplot := DEplot(SIRsys, [a(t), g(t), v(t), m(t)], t = 0 .. 100, g = 0 .. 0.211, [[a(0) = 0.17e9, g(0) = 0, v(0) = 4, m(0) = 2]], scene = [t, g(t)], thickness = 2, linecolor = red, stepsize = .1):

vplot := DEplot(SIRsys, [a(t), g(t), v(t), m(t)], t = 0 .. 100, v = 0 .. 0.1e4, [[a(0) = 0.17e9, g(0) = 0, v(0) = 4, m(0) = 2]], scene = [t, v(t)], thickness = 2, linecolor = red, stepsize = .1):

mplot := DEplot(SIRsys, [a(t), g(t), v(t), m(t)], t = 0 .. 100, m = 0 .. 0.1e4, [[a(0) = 0.17e9, g(0) = 0, v(0) = 4, m(0) = 2]], scene = [t, m(t)], thickness = 2, linecolor = red, stepsize = .1):

#
# Did no syntax checking beyond this point becuase I got
# really, REALLY BORED
#

L:=<<seq(i,i=0..100)>|<seq(B[i],i=0..100)>>:
#points:= {seq(B[i],i=0..100)};
 

Warning, plot may be incomplete, the following errors(s) were issued:
   cannot evaluate the solution further right of .33370860e-5, probably a singularity

Warning, plot may be incomplete, the following errors(s) were issued:
   cannot evaluate the solution further right of .33370860e-5, probably a singularity
Warning, plot may be incomplete, the following errors(s) were issued:
   cannot evaluate the solution further right of .33370860e-5, probably a singularity

Warning, plot may be incomplete, the following errors(s) were issued:
   cannot evaluate the solution further right of .33370860e-5, probably a singularity

SIRsys; a(0);g(0);v(0); m(0);

dsolve([SIRsys[], a(0) = 0.17e9, g(0) = 0, v(0) = 4, m(0) = 2]);

p1:=pointplot(L, color=blue, symbol=diamond):
display([p1,aplot]);

Error, (in plots:-display) cannot make plot structure from object with name DE1plot

Error, (in plots:-display) cannot make plot structure from object with name DE2plot