I'm unable to add my phone number to begin freelancing because I receive an error message that it's still associated with my old account. I deactivated my old account and don't even know what email address it was associated with. I'd really like to be able to work.
You appear to have misunderstood some important Upwork rules.
You are not allowed to have an old account and create a new account. Freelancers are permanently terminated from the platform if they are caught doing this.
You may work with Upwork Customer Support to resume using your first account, and work with them to shut down the new account you created. If you will work with them, you will not get in trouble.
I'm trying to open an account but my phone number is already in use and I can't create one without that... I probably opened one years ago and never did anything with it. I don't know the username or password for that pervious account but I want to open a new one with my current credentials.