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
http://ftp.jaist.ac.jp/pub/eclipse/technology/epp/downloads/release/2019-12/R/eclipse-committers-2019-12-R-win32-x86_64.zip
is suitable version for Mendix