You’ll need to create new partition table as a first step … as mkfs.ext4 step is the last one there. You might choose to use Disks application for preparing your Samsung T7. Or type within your Terminal app: sudo fdisk --list and locate Samsung T7 mount point (/dev/XXX). This post should help with your initial step: Talking about Librem 5 in French language - #89 by Quarnero. Please change bs=8M to bs=32M and from there use Disks, etc. Let us know if something unclear and/or post here initial sudo fdisk --list output so we can confirm which mount point took your external SSD.
Also, as related and helpful: https://linuxhint.com/gnome_disk_utility/.