I have found in the past with Aspire - that sometimes the data for a function key does not show up in WebPro, but in fact there is something there. So, when trying to assign a new or different function - sometimes that error comes up.
What usually works is to set the function key to None first, and make sure the data field is deleted as well (even if it already looks blank)
Apply changes to see if it accepts that change. Thne go ahead and try to program the key as you would like.