App Inventor just uses Google's account login. Since you're logged into your personal account, App Inventor is using that.
One way to do this is to open an Incognito window. In Chrome and Firefox, this option can be found under the 3 dot/hamburger menu to the far-right of the toolbar. An Incognito window should have no login cookies, and you can log into the correct account to use App Inventor.
You should also be able to go to https://myaccount.google.com and log your entire browser out of your personal account.