I have a photo of myself and I sent me model release for signing through Adobe Sign. I filled in all data, but I can't to fill Sign! It's active, but I can't type anything at all.
What problem it could be?
It can be all and nothing that it does not work for you. From your problem description it is quite difficult to determine what the problem is. I recommend, you do the model release the classical way, signing as the artist and the model and letting someone else sign as witness.