you have queued too recently. You can que again in 15mins.

sivistojko · November 2019

After accepting my invitation to random dungeon, I traveled to another location by shrine only to find myself out of the que and with this 15min time restriction?! Why?

Neoealth · November 2019

Because ZOS

DTStormfox · November 2019

Did you enter the dungeon and then leave it?
If so, you were probably marked as a deserter, hence you are 'punished' with a 15 minute time restriction.

Major_Lag · November 2019

OP, did you by any chance port elsewhere via wayshrine after accepting the queue pop but before getting ported to the dungeon by the game?
Doing that is Very Bad and causes all kinds of negative effects to happen, because it confuses the game as to where you should be placed.

For example, if you do that after accepting a Cyrodiil queue pop, you are stuck as both "being in the queue" and "not being in the queue" - can't queue again and can't port to Cyro either, only logout fixes that.
(edit: oops typos)

azjuwelz · November 2019

Had it happen to me yesterday--received the invitation just as I was going through a door, loading screen, then no queue and a penalty.

Very annoying because I don't think the loading screen was over 40 seconds.