can not delete readynas 102 snapshots

First of all, if you use the SMART snapshots, the monthly snapshots are never deleted.  So I always recommend using Custom snapshots instead - where you can set retention.

On the space reclamation - deleting a couple of snapshots often won't free up space.  Generally you get the most space gain by deleting the oldest snapshots first.  Often you need to run a balance afterwards to fully reclaim the space.

You could try deleting them all using ssh.  Though since your volume is very old, I suggest backing up your data, doing a factory reset, reconfiguring the NAS, and restoring the data from the backup.  You'll find that not only solves the snapshot issue, it should also improve your overall system performance.  Netgear has improved the btrfs configuration over of the years, but those improvements only take hold on a new volume.