I'm new user in maple programming and I trying to constuct a for loop but I taken this Error on my program! here is my program:

I have problem by serie solution Of maple I wanna to solve tehe following equation and the error happens I want to know whats this???

> sys := [s0*(diff(w1(x), [`$`(x, 4)]))+s1*(diff(w1(x), [`$`(x, 2)]))+s2*w1(x)+s3*(diff(f1(x), [`$`(x, 4)]))+s4*(diff(f1(x), [`$`(x, 2)]))+s5*f1(x)+s6*(diff(f1(x), [`$`(x, 1)]))*(diff(w1(x), [`$`(x, 1)])) = 0, t0*(diff(f1(x), [`$`(x, 4)]))+t1*Nx+t2*f1(x)+t3*(diff(w1(x), [`$`(x, 4)]))+t4*(diff(w1(x), [`$`(x, 2)]))+t5*w1(x)+t6*(diff(f1(x), [`$`(x, 1)]))^2 = 0];