What is causing this error?
When you see a lock icon on your profile or receive the error "participant limit reached" while playing on individual mode, it typically means that you were logged in on a guest account and got logged out. When logging back in as a guest, a new guest user ID is created that is different from your previous guest ID. This means the app is now recognizing you as a new participant and will not let you join the same profile.
How can I join my participant profile again?
In order to join your participant profile again you will need to contact the creator of your Experience. They may be able to change the settings on your participant profile from "Individual" to a "team" which would increase the number of participants allowed under your profile. Once they switch your profile to a Team and increase the number of participants allowed, you will be able to re-join your participant profile again with your new guest account.
You can follow the instructions in this article to contact The Creator of your Experience.