OEM unlock isn’t optional, you gotta unlock before you can install custom OS/recovery
That said, it should be available for the Snapdragon version of the S10+, but you maybe need to connect to a Wi-Fi network first!
I had the same phenomenon on my phone and also read in other threads about this issue.
But /e/ OS is only available for the international (global) version of the S10+
SM-G975F/DS (Global); SM-G975U (USA); SM-G975W (Canada)
That said, it maybe possible to install if there is a generic image … if you are the adventurer type.
Check that your heimdall is installed via typing into terminal:
heimdall version
other than that it’s essentially the steps as shown in piero’s post… other than flashing /e/ recovery instead of TWRP because that may cause issues.