Authentication templates use Meta's one-time passcode format. Their text and buttons can't be changed.
Meta only accepts Authentication templates in its own one-time passcode format: Meta writes the message, and you choose how the code is delivered, whether to add a security reminder and when the code expires. A custom message can't use the Authentication category.
Choose Authentication, then the One-time passcode type, in step 1 of the wizard.
For any other message, pick Utility or Marketing instead.
Still stuck? Send AVX-3403 to hello@theavoxai.com along with what you were doing when it happened.