Some applications don't have the ability to edit the app's provisioning schema representation enabled by default. You can override this with a URL feature flag documented here: https://learn.microsoft.com/en-us/entra/identity/app-provisioning/customize-application-attributes#editing-the-list-of-supported-attributes
Noted in one of the "Note" boxes in that section, the feature flag with ?forceSchemaEditor.. will unblock you for this.