Unable to map Salesforce username to Account Engagement user

Scenario 1: the Salesforce username is already mapped to another Account Engagement user
1) Look in Admin >> User Management and find the Account Engagement user that has the Salesforce username mapped to it
2) Look in Admin >> Recycle Bin >> Users and find the deleted Account Engagement user that has the Salesforce username mapped to it

Scenario 2: the Salesforce username matches the email address of a different Account Engagement user
For example, the following three records exist:
1) Account Engagement User A with email address email_a@example.com (not currently mapped to a CRM username)
2) Account Engagement User B with email address email_b@example.com
3) Salesforce User A with username email_a@example.com

If an Account Engagement admin edits Account Engagement User B's record and tries to select "email_a@example.com" as the CRM username, that username would not show up in the CRM username dropdown because Account Engagement is "reserving" that CRM username for Account Engagement User A.

Scenario 3: The Salesforce username is not being used in any other account nor is it in recycle bin.
1) Check if the Salesforce user sync is enabled for the account.
2) If yes, then check User's Salesforce profile is mapped to an Account Engagement role.
3) If not, then CRM username will not appear in the list while creating user in Account Engagement.
4) Map the Salesforce user profile and Account Engagement role, save the settings.
5) Create user and map the CRM username.

Scenario 4: Transferred user management to Salesforce

1) Check if user assignment is managed through Salesforce (refer to our documentation)

2) If so, ensure that the user has been assigned to a user group

3) Once assigned, wait 10-15 minutes and the user will automatically be created