Game won't let me activate Big Welcome Labrynth in Mirror match despite having fiend on field

Screenshot (1148)
Just lost a Lab mirror because it wouldn’t let me activate the Big Welcome I had set the turn previously with Lovely. Lovely was still on field, so her condition was fulfilled. The Lab field was also up on my side. Easiest won game of all time that I just couldn’t do anything about, so my opponent got to resolve 3 backrow without me being able to respond.

The game would prompt me to activate Big Welcome, and it would be highlighted, but it wouldn’t let me click on it to activate.**