You have already linked the maximum number of phone numbers allowed for this Business Account. Delete a phone number to continue or request additional numbers error.
Solution:
The max phone number limit is not specific to a WABA but rather applies to the business portfolio. Business portfolios initially allow up to 2 registered business phone numbers, but this limit can be increased to a maximum of 20. If your business is verified or if you initiate 1,000 or more business conversations within a 30-day period using templates with a high-quality rating.
Allso we can ask the customer to check if their business portfolio has already linked 2 phone number (Business portfolio -> waba -> phone numbers) or screenshot as well to confirm the same.
If that then only 2 phone numbers are allowed to be they can disconnect the other number in the business portfolio and use the new one.