Out of the easy installer,
it seems there are two specifics additional steps for the /e/ installation on the galaxy s9 :
after flashing TWRP
reboot to TWRP, do the wipe things,
and before installing the /e/OS_build.zip.
You have to install :
no-verity-opt-encrypt-samsung-1.0.zip
and
VENDOR-27_ARI9.zip
Now you can install the device specific image of /e/OS
build for the ROM of your specific device from :