The main preview for presets is switched off by default in LR Classic 7.4 following feedback, as many customers said they preferred seeing the change only in the Navigator panel and not the main Editor window.
To enable from the Lightroom menu, go to:
Edit >> Preferences >> Performance (tab) on Windows
Lightroom >> Preferences >> Performance (tab) on Mac
Put a checkmark in the box “Enable hover preview of presets in loupe”
Thank you very much