How to replace an item in the offhand of a player?

So Im trying to make it possible to use an item to enchant something in your offhand. On other items it works fine, but I want to be able to enchant a book and have it turn into an enchanted book, can anyone help? Here's a screenshot of the procedure: https://drive.google.com/file/d/1IKTBvh9ib-MY9uRZnKPepNyZ20i8x3MI/view?…