The Discourse software is designed to build community. In order to PM people you need to reach a certain “trust level.” That isn’t built by mere time since you registered but by active participation. You may notice you have “badges” that are assigned to you. Your “first like” or a post that gets “10 likes,” a link that gets 25 clicks and so on. Even reading posts counts to a degree but obviously signing up, reading and contributing are different levels. It’s designed this way so people don’t spam forums, message people out of the blue, etc.
I don’t know what trust level it takes but information about Discourse software can be found on the internet.
Note, it’s not keeping you from participating in the community, it’s keeping you from using the forum for private purposes.