Why can't I activate Rank-Up-Magic Admiration Of Thousands on duelingnexus.com?
However Rank-Up-Magic Admiration Of Thousands isn't working. Is there a reason why I have missed?


If you can't activate your RUM it may be because of it's requirements. From it's text, emphasis mine:


  Target 1 or more Xyz Monsters in each Graveyard, all of the same Rank; Special Summon from your Extra Deck, 1 "Number C" or "CXyz" monster that is 1 Rank higher than those monsters, and attach those monsters to it as Xyz Materials. You cannot Special Summon other monsters during the turn you activate this card. You can only activate 1 "Rank-Up-Magic Admiration of the Thousands" per turn.


This restricts you because:


You have to select at least 1 monster from each graveyard. If your opponent does not have one you can't play that RUM.
All the selected monsters have to be of the same rank. If neither graveyard contains valid targets you can't activate the RUM.
If you special summoned something else previously, you can't activate the RUM.


All these are possible reasons (besides a bug, naturally) for you not to be able to play the Rank-Up-Magic card.