Not sure what you are saying. If you tell me where to find that I can get the answer for you.
Log into the NAS web ui, and go to the logs page. Download the log zip file using the link on that page. Open Volume.log, and scroll down to the section that starts with === df -h === - which is towards the end.
This is what I have, which I do not understand if I have space, I can always clean out stuff or upgrade to a 8TB drive?
To clarify this - your disks have multiple partitions.
The largest are the ones holding the data volume (/dev/md127 in your screenshot). Those are 3.7 TiB each. Those partitions are then formed into the RAID-1 data volume.
The OS partition is a second partition on each disk. It is much smaller (only 3.7 GiB). That is also formed into a RAID-1 volume (/dev/md0). The OS partition contains Linux and the ReadyNAS application software (and the NAS boots from this partition). RAID-1 mirrors the contents on both drives (so anything written to the volume is written to both drives in parallel). That allows the NAS to boot from either disk.
Your data volume is about 80% full - so it is worth thinking about upgrading to 8 TB. But that's not at a critical point yet.
On the other hand, the OS partition is completely full. That is critical - since nothing can be written to it, there is a risk that the NAS configuration files will be damaged (resulting in an inaccessible volume). Upgrading the drives won't help - the OS partition isn't expanded, only the data volume is.
You have three options:
Contact paid Netgear support via my.netgear.com They can clean the OS partition remotely.
Back up all your files, do a factory default (which reformats the disks, and does a fresh factory install). Rebuild the NAS, and restore all your files from the backup.
Enable ssh, and use the linux command line to find the files that are causing the problem, and delete them. This does require some knowledge of linux, and if you aren't careful you can easily lose data. So I do recommend a backup before attempting this.
If you don't have a backup of your files, then I recommend taking care of that right away. USB backup drives are reasonably affordable (8 TB might be the right size, and would give you room for expansion in the future).
As far as fixing the problem, I don't recommend using ssh unless you already have some skills with the linux command line. I'd go with the factory default option instead. Though it is tedious, you don't have that much data on the NAS. It is guaranteed to fix the problem, and it will give you a completely clean file system, so there is no possibility that a damaged configuration file or other damage in the OS partition wil cause issues in the future.
A variation is to back up the data, get the two 8 TB drives, and do a fresh factory install on those two disks. Then restore the data to the new drives.
