OptN app - can’t create an account..

so when I'm trying to create an account in opt-in app it just says oops please enter a valid phone number um so yeah that's basically I don't know why I just can't create an account so I'm entering a like a valid Us phone number without country code with country code is plus sign without plus sign and I just can't create an account seems there is some kind of a glitch or I don't know why it's not working but that's yeah that's just my input