The top reason why number registration will fail is that there is an active WhatsApp account or app associated with the number. Not only do you need to delete the app from your device, but you also will need to delete the WhatsApp account fully.
To delete your account
1. Open WhatsApp.
2. Tap More options > Settings > Account > Delete my account.
3. Enter your phone number in full international format and tap DELETE MY ACCOUNT.
4. Select a reason for why you're deleting your account in the dropdown.
5. Tap DELETE MY ACCOUNT.
For details on account deletion, please refer here.
Here are some other common errors from Facebook:
#1005: Access Denied - Please wait [time] before trying again
Solution: Wait until the timeout is done and try again.
#1005: Access Denied - Unknown error: biz_link_info_invalid_payload
Solution: Reinstall WhatsApp and then fully delete it.
#1005: Access Denied - This number has not been approved for an enterprise. Please contact support for more info.
Solution: Fully delete WhatsApp from the device.
#1005: Access Denied - There was an error verifying this phone number. Please try again later.