Because True is not a number.

Well your program must be wasting a lot of time somewhere. I don't know how you approached the problem, but you might want to check that you're not going back and forth on cells you have already visited. Other that that, I don't have a clue because I have no idea what you did ¯\_(ツ)_/¯

My code works for the sample tests. However it won't complete in time when trying the hidden tests.

I thought that mazes without an exit could be the problem, so I wrote some code to check if the outside wall has a hole in it to escape the maze.
It still won't complete within the 12 seconds.

Any suggestions?