This is because you are using the product operator between the elements, instead of usinga mixed element:
from fenics import *
L = 1
nx = 5
ny = 5
mesh = RectangleMesh(Point(0, 0), Point(L, 2 * L), nx, ny)
P1 = FiniteElement("Lagrange", mesh.ufl_cell(), 1)
P2 = VectorElement("Lagrange", mesh.ufl_cell(), 2)
element = MixedElement([P1, P1, P2, P1])
ME = FunctionSpace(mesh, element)
d_c_mu_u_p = TrialFunction(ME)
fai, pot, v, q = TestFunctions(ME)
Next time, please read the guidelines: Read before posting: How do I get my question answered? - #3 before posting a question, as it is quite clear that your error has nothing to do with your PDE, and that you could reduce your example to ~10 lines, as I’ve done above.