Cannot add number in Sign In & Security

What is the exact error message? Are you using a number from the same country as that to which the account is being set?

Some contributors to this forum will reply with a definitive statement saying you absolutely must use a different number. What Apple says is in this support article: If you can’t create an Apple Account or see “your account cannot be created at this time” - Apple Support and is more nuanced, saying that if you see a message telling you to use a different number then you do need to use a different number.

“If you’re trying to create an Apple Account and see a message that says the email address or phone number is unavailable or can’t be used, then an Apple Account already exists that uses it. Try a different email address or phone number, or reset your password for that account ( If you forgot your Apple Account password - Apple Support ).”

The easiest way to find out if you can use the same number is to try it.

My personal comment: This bit about not reusing phone numbers makes no sense to me. Apple apparently may expect everybody using their equipment to have their own telephone (even very young children), and carriers to never reissue phone numbers to a different person. Even in this day and age people may share a telephone.