I’ve been periodically slamming my head into the wall for the past ~10 months about why a new install of MacOS 12 Monterrey wouldn’t connect to my NFS server. Turns out I had to inform the client to use NFSv4.

nano /etc/nfs.conf

#
# nfs.conf: the NFS configuration file
nfs.client.mount.options = vers=4

Saved nfs.conf and both command-line and GUI connections worked immediately.