I bought a preset two years ago, it was ok on previous versions of LR.
In LR Classic it works very bad and it is said that color profile is missing (in this preset color profile is vsco fuji 400h). I have the file with this profile in macOS: ~/Library/Application Support/Adobe/CameraRaw/CameraProfiles as Fuji 400H v2C.dcp
But as I guess LR Classic doesn't work with these files anymore. How can I fix this and use my preset again?
If the profile shows as 'missing' then this means you do have an approriate raw image selected, but Lightroom can't find the profile that was applied earlier. That means that Lightroom does not see your profile, even though it's in the correct place. Your preset needs to apply this profile too, so that is why it is in italics and does not work.
You could try the following. Move the profile from its current location to the Desktop. Start Lightroom, use 'Import Profiles' and import the profile again. I have successfully 'revived' some old camera profiles this way. If that does not help, then the profile itself may be corrupted somehow.