@hampton123 I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.
Issue: I'm encountering an issue with my API Management developer portal. Currently, the portal only permits sign-ins and sign-ups via B2C. Previously, I successfully tested the sign-up and sign-in system. However, after updating the gateways for both the endpoint and the developer portal to go with my company's custom domains, I encountered a problem.
Upon attempting to sign up, I'm presented with the message "You must agree to the terms of use" on the "Complete sign up" page. The sign-in system works properly, it's only an issue with the sign up page. There's no prompt or anything for any terms of use, so I'm confused on where the issue is.
Solution: Resolved by @hampton123
Navigating to my API Management service's "Identities" blade and pressing "Settings", there was no text for the Terms of use on user signup. Once I added some placeholder text, the terms of use showed up in the actual developer portal. I was able to accept the terms of use and log in with my account.
If you have any other questions or are still running into more issues, please let me know. Thank you again for your time and patience throughout this issue.
Please remember to "Accept Answer" if any answer/reply helped, so that others in the community facing similar issues can easily find the solution.