I tried to send myself a PM to try running my own solo adventure, documenting it PbP style, but the forum won't allow it. Is there another way people do this? Should I just create a PbP that is private and closed, and add only my character?

I'm confused, you want to run a PbP adventure but with you as the only member, full stop?

If so, why do you need to use a forum, why not use a google doc or word?

Yes, I want to solo my own adventure. I'm still learning, and want to do some trials.

I could use a text document, but I like the features the forum has. Persuasion 8

I mean, you don't need those forum features, you could just roll the dice yourself? The forum dice system is more of an 'anti-cheat' measure.

Of course I can. I could also use an online dice roller.

Alas, I like the format and tool of the forum. All I would need is the ability to PM myself.

I suppose I could just create a new account and PM that. But that's a workaround and not a feature.

If it's against the forum rules to do a PbP by yourself and the site gives you no functionality to PM yourself, then you cannot do a documented solo adventure using this site.

 I'm not actually sure a solo PbP is against tge rules. You can check through here: https://www.dndbeyond.com/forums/d-d-beyond-general/play-by-post/2069-pbp-rules-and-faq-please-read-first

Couldn't you just open the program in two different browsers and (using two screens if you have them) play from one browser as DM and the other as the player?