Three years later and I'm seeing the exact same issue in iOS16.2.

My colleague in another region can access and use App Store Connect without issue, but I cannot. If you have a VPN, try that to change your location. Apple doesn't seem to be aware of the issue so it might take a while to be fixed.

My colleague in another timezone is able to access App Store Connect without issues. (Looks like a bad deployment or CDN issue.) Try a VPN to access ASC from a different region.

Nope.

After much testing of offer codes, I have concluded that the entire implementation is a hot mess and I would not invest much time in them. If you use an offer code, you are purchasing in production (not the sandbox). This makes them impossible to test as your app (in dev or testflight) will be listening for purchases in the sandbox, but the offer codes use the production app. The only way to test is to renew a subscription in Settings > App Store > Sanbox Account > Manage and then select a subcription that has auto-renewed several times and has stopped renewing. If renewing a purchase works there, then maybe it works in production? Also, the developer who built this UI would be immediately escorted from the building if Jobs were still alive. What exactly is the user supposed to tap on to continue after entering the offer code? (Hint: Not "Done"!)