External access issues

HelloJBIncluda 

welcome to the Microsoft community, My name is Recep and I'll be happy to help you today,
I understand that your external users aren't able to log into your team environment, they all get the same error stating: AADSTS50020 user account {INSERT EMAIL} does not exist in tenant (our tenant) and can't access (teams) in that tenant

Please note that the account needs to be added as an external user in the tenant first. Sign out and sign in again with a different Azure Active Directory user account.

Also note that if your application is an enterprise application that requires user assignment, error AADSTS50020 occurs if the user isn't on the list of allowed users who are assigned access to the application.

To check whether your enterprise application requires user assignment:

1. In the Azure portal, search for and select Enterprise applications.
2. Select your enterprise application.
3. In the sidebar, select Properties.
4. Check whether the Assignment required option is set to Yes.

Solution: Assign access to users individually or as part of a group

Use one of the following options to assign access to users:

• To individually assign the user access to the application, see Assign a user account to an enterprise application.
• To assign users if they're a member of an assigned group or a dynamic group, see Manage access to an application.