Can you try using our sample app? I just checked it in my own org, and I do see the Sign Up button shown.
If you are still having trouble even with the sample app, make sure you confirm the following:
- Under Settings → Account, ensure that Interaction Code is enabled
- In your OIDC apps General settings tab, ensure that Interaction Code is enabled as an allowed grant type (this option will not appear until you enable the option under Settings → Account)
