Shifter shows reverse, car does not reverse. P3120?

Hi all, I’ve got my hands on a 2001 Prius that a coworker had sitting for 2+ years after the cat was stolen.

I just repaired the HV battery, but now having trouble getting into reverse. The shifter will go into reverse but the car won’t reverse. Park, drive, neutral and “B” work fine however.

As for codes, I pulled a P3120 and C1259. Also have the triangle of death. Currently looking for my J2534 cable from my Volvo-owning days to set up with techstream to get subcodes, but wanted to see if anyone had any ideas as to what the issue could be.

Thanks!