New user NGSolve coding issue

Hello,

I am new to NGSolve, I am trying to write the bilinear form and the linear form for the Stokes problem, but I get an error as follows
.
f = LinearForm(X)  
f +=InnerProduct(rhs,v)*dx #Use the code to calc the righthandside
f +=InnerProduct(U_ex,v)*ita/h*ds(skeleton=True)
f +=InnerProduct(grad(v)*n,U_ex)
f.Assemble()
---> 33 f +=InnerProduct(rhs,v)*dx #Use the code to calc the righthandside 34 f +=InnerProduct(U_ex,v)*ita/h*ds(skeleton=True) 35 f +=InnerProduct(grad(v)*n,U_ex)
NgException: In MakeLinearFormIntegrator: must not have TrialFunction
v is a test function not a trial function
Any help would be really appreciated
Thank you

How is U_ex, ita, h defined? Does it contain a trialfunction?

No, U_ex is the manufactured solution. The trial function is u and is not in any of the functions included in the linear form.

Could you maybe provide a minimal working example of your problem?