There is a ".ttc" file which I cannot delete from the System -> Library -> Fonts -> Supplemental folder.
Even though the .ttc file is in the supplemental folder, I cannot use the font. However, when I tried to reinstall the font, the font book tells me that there is a duplicate file but fails to resolve it. This is why I want to delete the .ttc file by myself, but it does not allow me to do so.
MacBook Pro 13″, macOS 13.6
FWIW, the Supplemental Fonts cannot be deleted or reinstalled. They are on the locked system volume. They are only used if there is a document that includes one of the fonts.
You've now been included in the mess Apple created by insisting a user shouldn't see, or use fonts not considered part of their region/language.
No apps written by Apple will show you active fonts that are not tagged for your region. The Affinity suite also won't show them. Others do, such as all Adobe software and the SoftMaker Office suite. It all depends on whether or not the third party vendor is using Apple's API to hide these fonts.
The problem you've run into is that despite the fact you can't see this font (and many others installed by the OS) in your apps, they're still active. That prevents you from activating a same-named font since it is then a conflict.
Where exactly do you want to use Athelas where you cannot do so? What app?
In TextEdit, do Format > Font > Show Fonts > All Fonts and see if it is on the list and select it to see if you can use it.
Cannot delete a corrupted font file in Fonts -> Supplemental
