Error using mupadengine/feval Invalid equations.

I am trying to solve differential equations,but getting the error:

Error using mupadengine/feval Invalid equations.

error dsolve>mupadDsolve (line 334)

T = feval_internal(symengine,'symobj::dsolve',sys,x,options);

error: dsolve (line 403)

sol = mupadDsolve(args, options);

error: test2 (line 42)

Psol(z)=dsolve(eqn,P0)

the variables v,alfa,n,A,keff,ns are defined and known before in the codes;

And there is an unknown parameter a included in P0;

I would like to get solutions with the unknown parameter a through dsolve;

I will appreciate for any help!