Creation doesn't usually go for declaring things 'impossible'. Some challenges have higher Difficulty Checks than others, that's all. In a way it's very Irorian about that.
Esta is trying to call upon the Power of Asmodean Friendship to restore Merrin's memories of being Mariona Durán.
In game terms, this might correspond to a base DC of 20,
+5 for Esta not actually being Asmodean Friends with Merrin,
+20 for Merrin not actually having the memories Esta is trying to restore.
Very little is 'impossible', in Creation, but there are some things you can't do just by rolling a natural 20.