Usernames must be 3 to 35 characters: letters, numbers, periods, and underscores, with at least one letter.
Your WhatsApp username didn't meet Meta's rules: 3 to 35 characters; only letters, numbers, periods and underscores; at least one letter; no period at the start or end and no two in a row; not starting with "www"; not ending like a web address such as .com.
Follow the checklist under the username field until every rule is green.
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Still stuck? Send AVX-3017 to hello@theavoxai.com along with what you were doing when it happened.