How to add an .icc colour profile to OSX 10.15

The folder /Library/ColorSync/Profiles does, or should exist on macOS 10.15.
(Note the / at the beginning, signifying the root level of the volume, the US spelling of 'color', and the correct capitalization.)

It's possible, or even likely, that the similar path inside the User account may not be there. E.g. /Users/<yourUserName>/Library/ColorSync/Profiles

You can create either of these folders, if absent, and the OS will use any profiles contained inside.

You can check the location of installed profiles with ColorSync Utility. In the Profiles tab, select a Profile in the Sidebar, and the location will be given as part of the information provided.

If the root level Library is really missing the ColorSync folder, then I would suggest reinstalling the OS, as you don't know what else is missing. Or restore it from your backup, assuming it used to exist.