EMS error: "This task does not support recipients of this type. The specified recipient is of type MailUser. Please make sure that this recipient matches the required recipient type for this task." when running New-MailboxImportRequest command.

I am trying to import a PST file into an archive mailbox using this command: New-MailboxImportRequest -Mailbox <user>-IsArchive -FilePath <file path>

When I do that, I get an error message: "This task does not support recipients of this type. The specified recipient  <user> is of type MailUser. Please make sure that this recipient matches the required recipient type for this task."

Any ideas why?