I read the other references to this error, but I don't see one specifically relating to an Apex function call. I deployed a Change Set with the Process, Custom Setting & Apex classes. The class has an @invocable method, and the process, as constructed in the sandbox, works fine.
The action itself did not come over in the Change Set, and when I try to rebuild it I see the class & method listed, as well as the parameter I'm setting in the process. I cannot save the action, however, because I am getting the "We can't find an action with the name and action type that you specified." error.
I'm going to try building a new Process that references the same method; will let you know how that goes. ... Nope - same result. Class/method/var(param) appears in flow, but Save() fails.