you have to drag the object under the base

omg! sorry I misread the initial post. mm..you have more than one object to the background? maybe near the name Empty Hourglass appears the word: INFO! in red? if yes, you have more than two selected objects and you have to buy another slot at 75k (look to the right under the list drawers)or remove an object to save the avatar

ok.. try changing skin and then reselect the none skin or push two or three times times the choose button