When creating a new custom scale or editing the name of an existing one, you might find that the same scale simply disappears from the list and is not an option to use.
What can happen is that if a scale has already been created with that name before (and was renamed, for example), the system will not be able to recognize it - this could be what is causing the issue.
The best way to fix this is to try adding a new name to the scale - for example, instead of "Satisfied scale", use "Happy scale".