Hello Sam Wheat,
Thank you for posting your query in the Microsoft Q&A Community.
Please be informed that having an Owner role on the Subscription attached to your Azure B2C tenant won't permit you to be able to manage the B2C tenant.
You need to be assigned a Global Administrator or some privileged roles within the tenant before you can. Follow this link for more information
Also, access within the tenant depends on how your user account was created in the tenant. Only users with work or guest accounts with the right permission will be able to manage the tenant.
Learn more about user accounts in Azure AD B2C https://video2.skills-academy.com/en-us/azure/active-directory-b2c/user-overview
Let me know if further assistance is needed.
Babafemi