TL;DR: Entra ID error AADSTS50020: “User account from identity provider does not exist in tenant”

The fix only. No explanation, no diagnosis. Read the full article if any of this does not match what you are seeing.

1. Read the error. Note the User account and the identity provider (for example live.com for a personal Microsoft account) and the tenant name. Is that the account you meant to use?
2. Wrong account or old session: open the link in a new InPrivate or Incognito window, or a different browser, sign out of everything first, and sign in again with the right account (work or school account, not the personal one).
3. Not invited: an admin of the tenant named in the error must have the person as a guest. In the Microsoft Entra admin center go to Entra ID, Users, find the guest, and check they exist and have accepted. If not, invite them.
4. Guest exists but still fails after the user's own account was deleted and re-created: reset the redemption status instead of deleting the guest. Open the guest, Overview, Reset redemption status (B2B collaboration tile), Reset.
5. Enterprise app with Assignment required set to Yes: assign the user or a group they belong to.
6. Only OneDrive sync app, after a move between organisations: close OneDrive, back up the registry, then delete this key (Microsoft's documented fix):

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Common\Identity

Do not reset the user's local Active Directory password. It has no effect on this error.

Do not delete a guest account just to "start fresh" if it has group memberships or app assignments. Reset redemption keeps them.

← Full article, with the diagnosis and the why