The relevant partition is ext4 and has my other operating system Arch Linux on it. I've tried so many different fstab configurations, but I just can't get Steam to accept it. Here's the relevant line in /etc/fstab:
/dev/disk/by-uuid/df502a85-859d-4fe6-9331-c3c65e7ae406 /mnt/arch auto auto,users,exec 0 0
I've also tried changing the user permissions of /mnt/arch so that my user can read and write, but no dice. Any idea?
EDIT: I'm on Ubuntu 20.04.2 LTS. I can mount the partition just fine, no errors. Only Steam gives me the error (in the title) when I try to add the drive's Steam library (/mnt/arch/home/[me]/.local/share/Steam/) as an additional Steam library folder in Steam on Ubuntu (Steam > Settings > Downloads > Steam Library Folders > Add Library Folder).