If you can see the shift interlock operating when you press the brake, that tells me that the electrical part of the interlock is operating normally which would lead to a mechanical issue. You can try to pop off the cable's connection at the shifter assembly, then pull on the cable end with a pair of pliers while holding your foot firmly on the brake and also with the parking brake applied also. If it comes out of gear the issue will be with the shifter assembly, if it still doesn't then the issue is with the cable or transmission linkage.
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Wednesday, March 24th, 2021 AT 1:15 PM