Question is if inserted microSD card recognized at all (on Librem 5 terminal app), therefore:
sudo fdisk --list
This might serve the purpose (be taken as starting point, only if no data on this microSD card):
sudo dd if=/dev/zero of=/dev/sd[x] bs=32M count=2
Etc.
Few perhaps/hopefully related posts you’ll find here: Talking about Librem 5 in French language - #97 by Quarnero .