A fix has been implemented and we are monitoring the results.
Posted Dec 04, 2020 - 12:33 UTC
Facebook team is currently investigating an error message: "Invalid Page Welcome Message for Messenger Destination Ads: The Message Text (page_welcome_message) is invalid for ads with Messenger as destination. (#1815471)", when users try to create an Ads in FB Ads Manager. You can read the full report here: https://developers.facebook.com/support/bugs/402624624219689/ - We will keep you posted as soon as Facebook resolve the issue.