Facebook page_welcome_message ads are invalid for ads with Messenger as destination

trying to run a "Traffic" ad for the Facebook bot server. To initiate a bot conversation, I need to use the "Start" button within the conversation with the posted ads and therefore need to use the "Structured Message (JSON)" option. I cannot post an ad as it always fails with this error

"Welcome message with invalid page for Messenger recipient ads: message text (page_welcome_message) is not valid for Messenger ads."

It is not clear what "page_welcome_message" is and where to install it.

I tried researching the FB community and tried to get in touch, to no avail - although I did find other people with the same problem.