In future, please follow the guidelines for submitting a bug report. In particular, the Psi4 version number, the exact input file, and a complete error message. When I try to reproduce the error, I get the exact line number for the problem. Including that information makes it easier for us to help you.
In this case, I can tell you the problem even without that information. wfn.gradient is the name of the method that returns the gradient. The gradient itself is wfn.gradient(). You need to include the parentheses in the frequency call.
That said, why are you calling frequencies twice, and why are you setting return_wfn=True in the frequencies call? The two frequency calls should give a nearly identical result, and you aren’t saving the wavefunction you get from the first frequency call.