When i put something on a tool rack using the book, it stays like that forever, i cant destroy the armourstand neither in survival or creative, i cant change it using the book itself and i cant get rid of it using /function armor_statues:admin/unused.

Is there a way to fix this that ive missed or is it bugged and i can never get rid of it?

You can use the Armor Statues book to fix this. You need to "unlock" the armor stand first, then set it to be not invisible.

As a small note, this issue has been resolved in the next upcoming patch for 1.17!