The first step to figuring out why you can’t send messages is determining who is at fault. Sometimes, there are issues with Instagram or Meta, its parent company, that have nothing to do with you. In those instances, there really isn’t anything you can do to fix the problem, and you just have to wait it out.

Especially when you consider that so many people across Instagram are having the same issue at the same time, it’s unlikely that this messaging problem was caused by something the users did.

Source: https://www.distractify.com/p/instagram-failed-to-send-message-how-to-fix