I've had this problem for a while now; I can't use any items directly from my inventory. I first noticed it when trying to use a Dye Kit on a pet. When I clicked the 'use item' button the message would come up as 'Oops! You do not own this item'. Since then, this glitch has happened with every item in my inventory; I can't use dye kits, squishies, or anything!

 

If anyone knew what was wrong with my inventory, I'd be glad to hear. Thank you! :)

Sounds like you are using some device other than a PC or a Mac. What are you using?

Internet explorer on a regular computer.

I would suggest using a better browser, like Chrome or Firefox. IE is notorious for misbehaving and causing problems. You could always try updating your IE to the latest version but that doesn't always solve the problems it causes.