Hi, ​@La Ligne!

There's probably something at Meta's side preventing you from sending messages (maybe some daily limit, number quality, necessary validation...).

Check your BM/WhatsApp Manager: https://business.facebook.com/latest/whatsapp_manager
It's possible you find more information there.

Nothing about WhatsApp ony page. It's just personal stuff (Instagram account, Facebook account). I guess I am supposed to create a pro page, but Facebook doesn't allow it...

That's probably it… Meta only allows businesses to use its Official API. This means that you have to set your accounts up with the Business Manager and even verify your business (this is easy, but you need to have a business website).

In this help article you find everything necessary to connect it with success: https://help.manychat.com/hc/en-us/articles/14959925356572-How-to-connect-your-WhatsApp-account-to-Manychat