It’s not clear to me what the intended behavior of the program is, but the direct cause of the error is that f_heat involves a TrialFunction. When using the a == L syntax for the first argument of solve, a should be linear in each of a TestFunction and a TrialFunction, and L should be linear in a TestFunction (and not include a TrialFunction). In the given code, L not only includes the TrialFunction component u_2 (through f_heat), but is nonlinear in it. (With the given code, I first get a different error due to temp=u_n in the arguments passed while defining sigma, because u_n is not scalar, which I resolved by passing u_n1 instead.)
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