If using administration level access within office 365 to access the mailboxes for migration the following PowerShell commands will need to be run to enable sufficient access to complete the migration.
Please note. Having control panel access or delegated admin for the mailboxes is not sufficient.
Even if you are able to access the mailboxes through the admin level account you will not be able to carry out the migration.
Please seek approval from your systems administrator before running these commands.
Before running the commands below you will need to make sure the user account you are using has Global Administrator rights within the Office 365 admin panel. Please allow 15 minutes for changes to propagate before attempting the commands below.
Please see the following link on how to obtain and use Powershell https://docs.microsoft.com/en-us/office365/enterprise/powershell/connect-to-office-365-powershell
To grant sufficient permissions for migration please execute the following remote PowerShell commands:
$cred = Get-Credential
$session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $cred -Authentication Basic -AllowRedirection
Get-Mailbox -ResultSize Unlimited | Add-MailboxPermission -AccessRights FullAccess -Automapping $false -User <AdministratorAccountName>
Please note: The command needs to be applied each time a new mailbox is created.
The text <AdministratorAccountName> above needs to be replaced with the admin accounts name