The Font Manager does not support system fonts other than the ones that are setup in the theme out of the box. The reason for this is because they are web safe
You will need to add your font as a custom font in the Font Manager > Custom Fonts. Please see the Font Manager documentation at https://theme.co/apex/forum/t/font-manager/101. The Custom Fonts instruction is not available in the documentation yet but the usage is straightforward.
If your font type is WOFF or WOFF2, you might need to manually upload your fonts to your server because WordPress and / or your host might not allow those file types to be uploaded via the browser. In that case, please follow the steps at https://theme.co/apex/forum/t/install-google-font-lato-locally/50332/25?u=christian_y