Troubleshooting permission to create new mailboxes
When configuring the connection to a target server, the connection wizard checks if the provided admin account is assigned the Mail Recipient Creation and Mail Recipients roles. If they are missing, the following warning messages are shown during the Configuration step (Fig. 1.):
Role 'Mail Recipient Creation' is not assigned to admin account
Role 'Mail Recipients' 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).
Even if the Mail Recipient Creation and Mail Recipients roles are not assigned to the admin account, this will not impact the migration process as long as you are migrating data to the existing mailboxes. However, if you want the program to create new mailboxes on the target server, you need to assign these roles to the used admin account. The program will attempt to assign them automatically (Fig. 2.), as well as other missing roles. Click Yes to proceed.
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.). If you are certain that this account belongs to the above role group, make sure that you typed the credentials correctly.
Enter the admin account 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.