The others are right, you have to be outside of the inn room before you can pop the fantasia. This may go for personal rooms in FC houses too. I usually do it outside the door to the FC house where I have access to all of my glamour gear within a short distance so no reason why you can't use it just outside the inn room then run in if you need to make any last minute tweaks.

You'll also need to take all your gear off before it will let you access the character creation, but I think the game prompts you to do this. Not sure why it's like that because it would be nice to see what your new character design looks like in your current or chosen glams.