This could be as a result of the new rule below:
Restricting DMs between teens and adults they don’t follow
To protect teens from unwanted contact from adults, we’re introducing a new feature that prevents adults from sending messages to people under 18 who don’t follow them. For example, when an adult tries to message a teen who doesn’t follow them, they receive a notification that DM’ing them isn’t an option. This feature relies on our work to predict peoples’ ages using machine learning technology, and the age people give us when they sign up.
Source:https://about.instagram.com/blog/announcements/continuing-to-make-instagram-safer-for-the-youngest-members-of-our-community