You want to rename your filenames according to your existing tags?
Your conversion pattern has to match your existing metadata.
You can only rename your mp3 to %artist% - %title% if both metadata tags %artist% and %title% are filled with data.
It the OK button remains grey, the converter isn't able to find your tag data and therefore isn't able to rename your mp3.
You could show us the tags for your "grey track" (pressing ALT + T).