If you are still unable to enable a cheat from the aforementioned list, it just means that you are not comfortable with the classic tank controls in Tomb Raider 1-3 Remastered. There is a noticeable delay between all movement animations. If you want to make Lara step forward and then backward, you need to wait until she has completed her movement animation before punching in the next command to step backward. In addition, you need to be walking (not running) while entering the cheats, so hold down the walk button.