Have you chosen the necessary font for your signature, but it isn't showing up on your Mac? The main reason for that is that this font isn't installed in Font Book on your Mac.
Please check the instruction below on how to fix that:
Step 1. Open Font Book from your Applications folder
or just press Command + Spacebar and enter it. Alternatively, you can locate it in the Main Menu.
Step 2. Then check if you have the necessary font in your Font Book by typing the font title in the search bar.
Step 3. Check if you see the necessary font type in the All Fonts list. If you don't see it or it's changed by another font from this font family, follow this guide on how to Install and validate fonts in Font Book on Mac.