Will Allah answer the prayer of someone who intended to repent but did not return stolen money? And is it permissible to frequently ask people to supplicate for one?

Theft is a grave sin, and it is among the major sins. One must repent to Allah for it by returning the stolen property to its owner, without the condition of informing him of it. Sincere repentance from sins and returning rights are conditions for the supplication to be answered.