(Sympy) Solving multivariate function with boundary conditions: 'Mul'/'Add' Object not Callable

Trying to implement a solver which looks like:

             sp.solve(U2(x,y,z),x,y,z, 
             ics={U2(h,0,0):0, U2(-h,0,0):0,
                 U2(h,0,L):0, U2(-h,0,L):0,
                 U2(h,W,0):0, U2(-h,W,0):0,
                 U2(h,W,L):0, U2(-h,W,L):0})

Where U2(x,y,z) is a very complicated function that includes, sp.sin/sp.cos/x/y/t... The hint type is a Union[Union[float, Mul], Any]. However, when trying to implement this solver I get the error that is either 'Mul' or 'Add' Object not Callable.

Maybe the choice of solver is not correct but I am not sure how to solve this multivariate function with boundary conditions.

Grateful for help.