Here are the elements to check:
Make sure that all templates in the flow are approved, and that all images and videos have been properly uploaded.
Ensure that a payment method has been validated on Meta. Without this, sending will not be possible as Meta will not be able to process the payment.
Check that your number is opt-in. To do this, go to "Inbox," then "All," search for your number, and verify if "opt-in" is displayed in the top right corner of the screen. If your number is opt-out, you can manually opt-in.
If you are using an outdated version of WhatsApp, it may prevent the test from being sent. Make sure to update your app.