If you are seeing below error while mounting NFS repository in Veeam backup & Replication server using Proxy VM.
Make sure you have given access to initiator (source machine) of NFS folder.
Also check the network connectivity incase if you are using gateway server to mount the NFS then make sure gateway server is able to reach both VBR and NAS device.
Make sure there is no network latency between VBR, proxy VM and NAS device.
you can try to map NFS folder in proxy VM directly then try to mount in VBR server.
failed to connect to the NFS shared folder X.X.X.X:/volume/XXX: gateway or NFS server/shared folder may not be available.