Center of the flange 13 1/2" not 10" or 12" what do I do?
If the center of the flange on the floor is 13-1/2" from the wall where the tank goes and the new toilet is 10" or 12" then that is fine. There will just be a gap behind the toilet. If the toilet was set to 13-1/2" and the flange was at 10" or 12", then that would be a different story.