Unable to import preset. All items were already imported.

Hey! I had the same problem. Make sure to import in the profiles that are associated with the presets. This is the only thing that helped me.

In the presets panel,  lick on the + symbol, then manage those presets, you probably will find the missing ones are not turned on. One of several settings that probably reverted to default when you reset the preference file.

Since you downloaded and sucessfully installed the preset yesterday it is still on your computer. You can find it within Lightroom Classic by going to the preferences>presets tab and selecting the buttons in the screen shot... these buttons will show you where the preset is located on your hard drive and you can reinstall it. Where they are located depends on whether or not you have the option on to "store presets with this catalog". That is the thing to consider when Resetting Preferences - once reset you have to reestablish your preferences.