Cannot Apply a Transmogrification Appearance

If you are unable to apply a transmog, check the following:

1. Disable your addons. 
2. Mouse over the equipped item and check if it says "This item cannot be transmogrified". By design, some items can't receive an appearance.
3. Verify your character is eligible to apply the appearance you selected. Some appearances have some requirements before you can use them.
4. Check your character for buffs which apply a costume and remove the costume.
5. Make sure the item you are wearing is the right armor class for your class.
6. If you are using a previously created transmogrification set, try to apply the appearances one piece of gear at a time. It will help you find which item or appearance is causing the issue.
7. If you transmoged your weapon into an artifact appearance, try to change this appearance first for a non-artifact one. When try to change other appearances.

If your issue persists, reset your user interface.