Answer
That means the view genuinely isn't connected to a Logical Unit — custom fields require the underlying view to have a connected LU with an OBJKEY column available, not just to display data. If the view belongs to a localization or third-party/custom component, you'll need to check with whoever built that component (or IFS, if it's a localization) about whether it can be enabled for custom fields, since that's a structural property of how the view was built, not something you can add from the custom-fields screen itself.