Hi David,
Welcome to the Box Community, I'm happy to help!
I checked your profile and I can see that your email is indeed linked to a Personal Free Box account. If you need to migrate this account into an Enterprise Box, the Enterprise Admin (or a Co-admin with permission to manage users) need to manually add you in the managed users list in the Admin Console > Users and Group menu. This will prompt an invitation for you to join their Box Enterprise account as a managed user.
Once the Admin has sent the invitation, you have to login to your Personal account in Box.com and click the 'Accept invite', once you've accepted, your account will be rolled in to their Box Corporate instance, and should be able to use your SSO login to access Box moving forward.
This process was further discuss in this article: https://support.box.com/hc/en-us/articles/360044195233--Invite-Existing-Box-User-Message-When-Adding-User
Let me know if you have any questions and I will do my best to help you!
Regards,