Troubleshooting ApplicationImpersonation role assignment
When configuring the connection to source and target servers, the connection wizard checks if the provided admin account is assigned the ApplicationImpersonation role. If the role is missing, the Configuration step fails (Fig. 1.), as the program is not able to access user mailboxes. You also get the following error:
Role 'ApplicationImpersonation' is not assigned to admin account
If the program is unable to check the roles assigned to the admin account, it means that the used account does not have the View-Only Configuration role assigned (learn more).
To continue with the configuration, the program will attempt to assign the ApplicationImpersonation role automatically (Fig. 2.), as well as other missing roles. Click Yes to proceed.
Some of the management roles are optional if certain conditions are met. Learn more here.
If the program is unable to assign the missing roles, it means that the provided account doesn't have appropriate rights to do so. You will be asked to enter credentials of an admin user who is a member of the Organization Management role group (Fig. 3.) or, if you're connecting to the source Office 365, who is assigned the Global administrator role. If you are certain that this account belongs to that group, make sure that you typed the credentials correctly.
If you intend to provide the credentials of a Global administrator account that has the multi-factor authentication enabled, make sure to enter the app password instead of the regular Office 365 password. Otherwise, the program will not be able to use this account to assign the missing roles. Learn more.
Enter the UPN manually or click Browse to choose another account from the same domain. Provide a valid password and click OK. The missing roles will be assigned, and the Configuration step will finish successfully.
Note that only the credentials specified in the Admin account step are used to connect to the source or target server. Credentials provided in the Configuration step are only used once to assign the missing roles. Therefore, you will not be asked to provide the credentials of other admin users again as long as you use the same admin account for the migration process.
If you don't want the program to assign any roles automatically, you can do it manually, following the steps described in this KB article.