I have no idea if CYAN / Gimp cyan plugin works correctly in MacOS with a recent Gimp installation, however the message you get is because you have not set the default profiles - bottom of the Gimp cyan dialogue The drop down selection menus.
As a demo, new update to Gimp 2.10.30 (kubuntu 20.04) and a little nuisance when opening the image. The Gimp developers must have changed something, thankfully CYAN is still working.
Three minute video on what you might see: https://youtu.be/695-t0o97Qs
Just a reminder: CYAN is also stand-alone for converting RGB <> CMYK
As a demo, new update to Gimp 2.10.30 (kubuntu 20.04) and a little nuisance when opening the image. The Gimp developers must have changed something, thankfully CYAN is still working.
Three minute video on what you might see: https://youtu.be/695-t0o97Qs
Just a reminder: CYAN is also stand-alone for converting RGB <> CMYK