Error program_unavailable

NFS isn't registered on this server

The portmapper answered but has no NFS or MOUNT service registered for the version the app asked for. NFS is probably enabled but not running, or the server is NFSv4-only.

What this means

The portmapper on port 111 is running — so the server is up and RPC works — but when NFS Files asked “where is NFS version 3 / MOUNT version 3?”, the portmapper said no such program.

Causes and fixes

1. NFS service enabled but not started. On Linux, sudo systemctl status nfs-server and rpcinfo -p on the server; on a NAS, toggle the NFS service off and on.
2. NFSv4-only server. NFSv4 doesn’t use the MOUNT protocol, so a server with NFSv3 disabled registers no mountd. Set NFS Version → NFSv4 in the app’s Advanced settings.
   • TrueNAS: System → Services → NFS → Advanced → Enabled Protocols. If only NFSv4 is ticked, either tick NFSv3 as well or use NFSv4 in the app.
   • Linux: /etc/nfs.conf [nfsd] vers3=n means v3 is off.
3. A stale portmapper. After reconfiguring NFS, rpcbind can hold an out-of-date table. Restart the NFS service (or the NAS).

See for yourself

Inspect Server on the server card shows the RPC table — the same output as rpcinfo -p. If nfs is there but mountd isn’t, that’s case 2.