How old is this tank? Looks very new.
What are your water peramiters? Please post
How did you acclimate the nem?
By the picture I’m guessing this is a very new and steril tank. Anemones do not do well in new tanks and are not recommended to add till after a 6 month mark at the least.
I would take the anemone, pick it up, turn it on its foot and see if it will attach to the rock. Turn off all power heads during this time to let it foot.