Have a check on the Exchange online admin that you used. Mark sure it is contained in the “Migration” permission role.
You could create a new permission group which contained the "Migration" permission, then add your admin account to this group:
Please note: Exchange 2010 isn't supported now, if you want to create a hybrid with Exchange online. I would suggest you migrate mailboxes to Exchange 2016 first, then create hybrid with Exchange 2016.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.