@Asher B ,
This issue has multiple possible causes and resolutions, including the following.
Cause 1: Someone has already set up Teams for you.
Resolution 1: Look for an invitation in your email Inbox. Alternatively, try to sign in to Teams from a private or incognito browser window.
Cause 2: Your email address is tied to both a work and a personal account.
Resolution 2: Try to create a Teams account in a new private or incognito browser window.
Cause 3: You're trying to sign up for your work account by using a free tenant.
Resolution 3: Use your organization's tenant for this account. For personal use, create a free Teams account by using a personal email address.
For more details, please refer to this article.
Besides, many people discuss this issue in this thread. You could refer to what Christopher said.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.