Please remove inactive volumes in order to use the disk

Going over 90% capacity isn't a good idea with btrfs - though the usage might not be at the root of the problem - the rebuilding messages in your screenshot suggest something else was also wrong.  You should download the full log zip file, and look at the disk health (disk_info.log).

 

If you have a backup, the quickest approach is to do a factory default, rebuild the NAS, and restore the data.

 

If you don't, then I suggest using netgear paid support (my.netgear.com) for this.  You might need their data recovery service.  https://kb.netgear.com/69/ReadyNAS-Data-Recovery-Diagnostics-Scope-of-Service

 

Alternatively, if you can connect at least two (ideally all three) disks to a  windows PC with SATA or a USB adapter/dock, you could try ReclaiMe data recovery software.  https://www.reclaime.com/

 

 

Your firmware is also very old, and should be updated after you regain access to your data.  You'd need to do that in steps (upgrading to 6.5.2, and then to either 6.9.6 or 6.10.3).