Hi,
Same error (Signature verification failed) and it took me a few hours of trial/error and reading to understand and get past this.
In fact the problem comes from the reboot after the installation of /e/OS Recovery.
It says: 7. Manually reboot into recovery:
This reboot is difficult. If you miss it the recovery you downloaded is deleted and the next step cannot be validated.
I found the way to reboot correctly by reading the Lineage OS installation procedure => Install LineageOS on beyond1lte | LineageOS Wiki
IMHO it would be nice to specify this point in the e/OS installation instructions => https://doc.e.foundation/devices/beyond1lte/install
When rebooting after downloading the recovery you will see a different screen with the e/OS logo.
Attached a screenshot of the OS installation. (Not necessary to wipe data twice.)
Of course you have to be sure to have the right model SM-G973F (or SM-G973F/DS). SM-G973U is not good (no OEM unlock).
Cheers,