As the title describes, the variables in the SCSS for the flag icons are in uppercase:

https://github.com/coreui/coreui-icons/blob/master/scss/flag/_variables.scss#L3

But the SVG files are in lowercase:

https://github.com/coreui/coreui-icons/tree/master/svg/flag

This results in this error when trying to transpile the SCSS to CSS:

Syntax Error: ModuleNotFoundError: Module not found: Error: Can't resolve '../svg/flag/cif-AD.svg' in '/project/node_modules/@coreui/icons/scss/flag'

Changing the variables to lowercase fixed this for me.