Ive tried the restore option but I keep getting a unknown error 21 and I don't know what to do HELP

My sons iPhone 7+ is stuck on the apple logo for days. We've tried force restarting it and with the restore option it takes a LOT of tries to get it to the screen to restore and when it goes through the steps I get "unknown error 21". My Mac is completely updated. My iTunes is updated so I'm not sure what else to do.