What phone? Different phones have different contacts apps. For example - on my Pixel 2 XL when I open a contact (in the Google-provided Contacts app) and tap the 3-dot menu in the top right that menu does have a set ringtone option. Some phones or contacts apps have this in the actual edit contact screen.
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See this link for instructions on how to join Android Central.