Problem to solve ddt(U)+laplacian(nu,U)=f
 

I'm going to solve the below equation where U is the velocity field and f is a volume vector field with the dimension of m/s^2.

fvVectorMatrix UEqn
fvm::ddt(U)- fvm::laplacian(nu, U)+f

By using a solver like 'PBiCG' i get the error that i need a symmetric matrix solver.
After choosing a symmetric solver, i get the error that :
keyword agglomerator is undefined in dictionary
From function dictionary::lookupEntry(const word&, bool, bool) const
in file db/dictionary/dictionary.C at line 402.

And when i use an implicit approach ( fvm::Sp(1,f) ) instead of 'f' , i get another error about incompatibility of f and U.

I would appreciate if somebody can tell what is wrong here.

You didn't choose 'a' symmetric solver as in an arbitrary one, but specifically GAMG, right? And then you get the error "keyword agglomerator is undefined in dictionary". May I suggest adding that keyword to your solution dictionary?

Thank you very much, it works.
Always i thought there is a problem in my code,So not concentrated on somewhere else! Thanks.