PokeMMO currently requires a Java Runtime Environment version 11 or higher.
Windows/macOS users are recommended to use the PokeMMO installers found on the Downloads page.
If this is a new issue, reinstalling PokeMMO from the above link may resolve it.
Manually installing Java
If you are unable to use the installer, you may manually install a compatible Java Runtime Environment from one of the following sources:
If you still require assistance, please open a ticket on https://support.pokemmo.com