This has been a bug for literal months and they have not fixed it yet, sometimes your inventory will become glitched. You can not move items in your inventory, your client sees that you can but the server has soft locked your inventory meaning if you move blocks from slot 2 to slot 3 you can not place those blocks AT ALL. Your client sees them but the server does not. The only way to use any item again is to update your inventory by opening a chest or the shop. You can not put on new armor you can't offhand anything you are basically instantly out of the game. Cube craft FIX THIS STUPID BUG OR I AM NOT GOING TO BE PLAYING ON THE SERVER ANYMORE.