I know the problem. I had Gimp 2.8 in this old OSX virtual machine and later updated to Gimp 2.10. For some reason there were files and folders missing. I remember adding a plug-ins folder. Should not happen but it did.
Providing everything else is working.
Open Gimp and look in the Gimp 2.10 menu then Preferences.
Bottom of Preferences dialogue is a folders section. Expand that, go to the palettes section and look at the top entry. That is where your palettes go. Remember that location.
In Finder go and find that location, GIMP/2.10
In that 2.10 section make a new folder File menu > New Folder and name it palettes.
Then Open Gimp again an see if it works
This is how it goes in the old VM https://youtu.be/I5l8hkemKgw
Providing everything else is working.
Open Gimp and look in the Gimp 2.10 menu then Preferences.
Bottom of Preferences dialogue is a folders section. Expand that, go to the palettes section and look at the top entry. That is where your palettes go. Remember that location.
In Finder go and find that location, GIMP/2.10
In that 2.10 section make a new folder File menu > New Folder and name it palettes.
Then Open Gimp again an see if it works
This is how it goes in the old VM https://youtu.be/I5l8hkemKgw