In a newly created ODM Migration Project, when trying to to discover on all accounts, task failed to discover accounts with this error :
UserPrincipalName not found for Tenant sgptoys.onmicrosoft.com.
Provide an active UserPrincipalName through Project > Accounts > Dashboard > Configure Connections.
Verify the UserPrincipalName and save.
Upon verifying from source tenant, many accounts actually do exist.