Ah, OK, I can see where this problem is coming from now (and why it didn’t affect me on my own Mojave machine). Try installing git on your computer. https://git-scm.com/
Explanation: we use an application called git to switch between versions of PsychoPy at run-time. We check if this is installed first, but it seems that the process of checking is no longer working under Mojave. The result is that PsychoPy thinks git is installed and proceeds to try and use it but falls over.
Thanks for getting a full error message to us. That will help us fix the issue more gracefully.
And do let me know if there are further problems after git is installed