@Preben Alsholm 
Thank you very much for your attention to my problem.

I use the parameter L and problem is solved.

@Preben Alsholm 

If i understand corrrectly the values of the functions U and y are very small and close to 0?
That's why you try to put in the system parameter L ?

But if i use only that string: res:=dsolve({sys,cond},numeric,approxsoln=[U(xi)=xi,y(xi)=(1-xi)*(xi-0.8)]); could i get some graphics of U and y ?

@Rouben Rostamian  
Thanks for your attention. Yes,there is. Because this is the task about free oscillations of a pipe and it's MUST HAVE nontrivial solution.I found that information in books.But I can't find the solution there.