My number won't save

The screen says why under the box. It is one of three things:

• "Start with the country dial code": type the number with its country code, like +971 50 123 4567, with no zero after the code.
• "WhatsApp does not work on a landline": use the mobile number WhatsApp is on.
• "That does not look like a mobile number": check the digits.

A number typed without its country code can still work when your account's country is set on Settings, but the country code is the sure way. See Your country: what it changes.

If it still will not save, you may be on the Free plan: the WhatsApp button is part of Pro.