I solved this issue by editing the .json file without using AMAZON.LITERAL here is the guide to do it: https://developer.amazon.com/docs/custom-skills/literal-slot-type-reference.html

I believe there is also a pull request made by someone else in MMM-MirrorMirroOnTheWallSkill git page. He includes his new json file for interaction model.

I also have the correct json file if you need it.

Good luck!