As of Mendix 8.0, it requires JDK 11 as listed in requirements here Mendix 8 system requirements. However, it is automatically installed with Mendix Studio Pro.
Please verify if JDK 11 is installed in your system and then in Mendix 8.5, go to Edit –> Preferences and check that correct JDK directory path is specified here (something like JDK 11.* hotspot)
Also see this guide to Debug java actions in Eclipse. Hope it helps.
To configure in Eclipse settings, please see attached picture below. Also this https://docs.mendix.com/refguide/using-eclipse from Mendix documentation
is suitable version for Mendix