Troubleshooting

Port conflicts

Make sure nothing is running on ports 2049, 32765 and 32767. If there's another NFS server already running, anylinuxfs will not work.

You should always mount individual partitions (e.g. /dev/disk4s1) not whole disks (/dev/disk4) unless they really don't have any partition table (which might be the case with LVM/LUKS containers sometimes).
Check your mount flags (e.g. the subvol flag from demo is specific to btrfs, make sure you don't use it with other filesystems)

Check file owner and permissions with ls -l and adjust accordingly. Typically, your macOS user won't have write access to your drive out of the box so you need to write files as root or first prepare a target directory writable by everyone (chmod 777).
To bypass Unix file permissions and make files appear to be owned by the current macOS user, use the --ignore-permissions flag (e.g. sudo anylinuxfs /dev/disk0s6 --ignore-permissions). This squashes all UIDs/GIDs on the NFS export and sets the noowners NFS mount option. Equivalent to manually setting --nfs-export-opts rw,no_subtree_check,all_squash,anonuid=0,anongid=0,insecure -n noowners.
If your drive appears mounted but you cannot browse any files (or the volume folder appears empty), it might also be a permission issue. When you run ls -ld /Volumes/<your_drive>, you will see something like drwx------. This can be fixed by running sudo chmod go+rx /Volumes/<your_drive>. Beware that this will effectively allow any user to browse your files though (at least in the root directory – other sensitive files are often protected individually). If this is not what you want, just use terminal commands with sudo for any file operations.

If you get fcopyfile failed: Operation not permitted, it can actually mean the file you're trying to copy has the quarantine attribute set (can be removed with xattr -d com.apple.quarantine <filename>)

Accessing disks might require Full Disk Access permission (although you should get pop-ups that let you allow access case-by-case)