clarky000@gmail.com
You can assure via echo $G_DISTRO_NAME.
If it’s Stretch, Kodi 18 should be available in the APT repos and installed via dietpi-software.
If it’s Buster, as said there is not yet a Kodi version available that supports the RPi GPU, although it is expected soon. Until then there are 3rd party packages available that can be tested: https://github.com/MichaIng/DietPi/issues/3031#issuecomment-518038469