Developers who are not selling anything do not have a requirement to make a public contact method available on their page. If they have comments open, you can leave a comment, and if they wish to respond, they will. You can also try looking up their profile to see if they have any other contact methods available or social media accounts.
If you have purchased something and need help with that product, you can contact Itch.io support, and they can forward your request to the developer. However, we generally respect the privacy of developers and will not forward non-critical requests unless absolutely necessary.