@dharr Thanks a lot for your hint. I try to solve this error using your hints. best regards.

@dharr Thank you so much.  Actually n:=2  doesn't affect the code. I forgot to eliminate this line. With your modifications, does the code run successfully and solve the problem? In that case, if you applied these modifications to the code, is it possible to share it? Thank you again for your kindness and time

@dharr Thank you so much for your answer. The error still exists. This is the whole code. It should give the result for "m" which is actually omega, and plot some curves. Thank you for your time. code.mw

@C_R Thanks a lot for your big help. the problem was the same thing you mentioned. I am working to drive the correct equations.

@dharr Many thanks for your answer and time. As I checked the equations again, I noticed that there was a mistake as you said.