Nano Memory card does not show up in Finder nor Disk Utility I have a Nano Memory card working fine. I know it's fine because it contains a bootable ubuntu OS that is being used in a Raspberry Pi. 

When I plug it to my Macbook Pro (Mojave 10.14.5) it does not show up in Finder nor even in Disk Utility.
Working as designed.  macOS doesn't support the linux filesystems that linux uses to boot.  The one I use for booting is ext4.

check what filesystem you formatted flash driver with. 

For ext4, you can install one of these drivers. 

http://osxdaily.com/2014/03/20/mount-ext-linux-file-system-mac/

https://www.paragon-software.com/us/home/extfs-mac/#