I used minimal ADB and fastboot to check if the debugging mode is really on and if the device isn’t unlocked. I feel more confident by using this little tool to install a custom ROM.
Maybe you can find and download the Android 7 Nougat firmware via a tool like samfirm or here (https://updato.com/firmware-archive-select-model/?exact=1&q=GALAXY%20S7) to downgrade the device before retrying to install /e/?
Or you try if there is a difference by using minimal ADB and fastboot at first. But I don’t really think so.