Thanks for reaching out.
Which version of Lightroom Classic and macOS are you using?
Please try deselecting "Use Graphics Processor" in Lightroom's Preferences > Performance, then relaunch Lightroom and let us know if it helps.
Are you clicking on the presets in the Presets panel, or are you just hovering the mouse over a preset and expecting to see a preview of the preset on the large Loupe image? If so, ensure the option Preferences > Performance > Enable Hover Preview Of Presets In Loupe is checked.