Can I try playing chess with myself, just for a little training?

Yeah, maybe... But it isn't the same thing, I guess

If you mean you play this account against another of your own accounts (even in unrated), this isn't allowed.
You can however, just turn off engine lines in the analysis board and play against yourself this way. Bonus is that when you are done, you are already in the analysis board to review the game!
https://support.chess.com/article/596-can-i-have-multiple-accounts "[not allowed to] Play the accounts against each other, even in unrated games"

I mean, if I do, I will play unrated matches, I wouldn't just use it as an easy way to get elo, it's just for me to get a better unterstanding of chess moves. Is this against the rules or can I do it?