Can’t customize controls

I was able to activate the U1 for my TV and Comcast remote, but it would not allow me to customize controls.  I held the desired function for at least 5 minutes (seems way too long) and the U1 said "learning", but there's no evidence that it was as it didn't adopt the function of the Comcast remote.  Guidance?  If I can't get it to work, it is going back.