After you installed PBS, you need to install the CIFS and NFS packages (libnfs12-utils and cifs-utils if I'm not wrong), then create your mountpoint somewhere and set the user and group owner to "backup:backup"I am interested in how you did this. I too want to use my Synology with PBS and a NFS share. How did you get Proxmox to see it?
Create your share in the Synology and set NFS permissions to the PBS ip, but connect the share in CIFS first, then add a new datastore in PBS where the CIFS share is mounted
It will go in timeout if you created from from the web console, but that's normal, after some minutes it should appear as a new datastore
Before rebooting, set in the fstab the same NFS share you used with CIFS then do a clean reboot, everything should be working fine (including prune, unlike CIFS)