ReadyNAS 214 volume is inactive or dead.

patelshivrajs​ said: A technical support from third party told me that actually a last slot is open and installing a new drive in it going to solve a problem instantly as the storage is full. I doubt this as a week back i had deleted 2 TB of data. Also he said that reinstalling the OS will solve the issues. What are your views??

Bad advice.  The size in your screenshot is the max size (capacity) of your RAID-5 volume.  It does not tell you how much data or free space is available on the volume.  The "inactive status" means that an error is preventing the volume from being mounted.

It looks like you have one failed disk (the one that is empty).  It's possible that a second one has also failed - in which case you are likely going to need RAID recovery to get your data (and that might not work).

It is also possible that we can forcibly remount the volume.  That requires use of tech support mode (and the linux command line).  There is risk there, particularly if you have never used the linux CLI.  So even if that is possible, you still might want to use RAID recovery.

The logs would give more info, so it is best to wait for Netgear to resolve the PM problem.