This is usually due to using a selection color on MacOS that doesn't appear on Final Draft. To fix it:
- Close FD if it’s open;
- On the Mac, go to System Preferences > General (Big Sur and Earlier) or System Settings > Appearance (Monterey or later)
- Next to Highlight color, select the option you’d like to use (the only option that will not work is Graphite);
- Close out of System Preferences;
- Open FD, and try to highlight text.