One can think of many reasons, but the first guess is that the problem has not been formulated adequately (ill-posed; I would recommend checking all the boundary conditions as a first step). For NS equation, this could also be related to a pressure point constraint. See here, if this helps-

https://www.comsol.com/community/forums/general/thread/54451/

Hello Amirhosein,

I concur with N.N. Misra; the problem is most likely ill-posed. Further to his link you may also want to look at this article by COMSOL:

https://www.comsol.com/blogs/improving-convergence-multiphysics-problems/

I've always found the following generic tips very useful:

http://evgeni.org/oldfish/Top_nine_hints_for_working_with_Comsol

Hope you manage to solve the issue - keep us updated.

Jules

Thanks Mr Hammond,thanks Mr Misra

 I'd seen the comsol website But it was not useful and I did not understand what can be due to errors

Changes listed on the comsol forum did not work and not effective...