Hi there, @evansmuriu291.
Let me share some ways to get rid of this error message and so the new user can register to the QuickBooks Online account.
The invitation link may have already expired, that's why it's asking the user to enter a valid phone number with the area code. To fix this, a new invite needs to be sent so the user can enter the phone number without the error message. Also, make sure to use a phone number with the same region as your QuickBooks version.
If you get the same result, I recommended doing some browser troubleshooting to fix unexpected behavior, like an error message. Things like cache or an outdated browser can sometimes be the culprit for such an issue. For information on this and recommended steps, please see the following article: Troubleshooting browser problems.
I've included some resources on managing users, and tutorials for some How Do I tasks:
As always, I'm just a post away if you need any additional help. Take care.
