WhatsApp will not hold one number twice, so if you see this, something still has it. There are two causes and the message does not tell you which, which is why this trips people up.
Cause one: it is still live on a phone
Most common, and the easiest to fix.
If the number is on the WhatsApp Business app and you want to keep using it there, you do not need to release anything. Use the first button on the connection screen, “My number is on the WhatsApp Business app”, described in connecting your number. That is the whole point of that option. It exists so you do not have to choose between the app and ServiceM8.
If the number is genuinely finished with, delete its WhatsApp account on the phone: WhatsApp, then Settings, Account, Delete my account. Wait three minutes, then try again.
Deleting the account erases that number’s chat history on the phone. Be sure before you do it. If there is any doubt, use the connect option instead. It keeps everything.
Cause two: a previous provider still holds it
Less common, more annoying, and not your fault.
If you have ever used a chatbot, a marketing agency, or another messaging tool with this number, it may still sit inside their WhatsApp Business account, even if you never finished setting it up with them and even if that was years ago.
The error offers a link to a WhatsApp Manager you will not have access to. That link is still useful: the account number in its web address identifies the account holding your number. Send that to whoever you think it is and ask them to release it.
If you cannot reach them, or they no longer exist, Meta’s support can release the number once you show it is yours.
If you are not sure which one you have
Email us. Send a screenshot of the error including the address bar, and we will tell you which of the two it is. It is a five-minute answer for us and a frustrating afternoon otherwise.