Unable to take backup of one server and backup is getting failed due to VSS subsystem issue.Attached screenshot for the same.Kindly suggest

lmgtfy.com/

With no other information, in my experience the 3rd one (High IO on the disk) is by far the most common cause. Its also the hardest to fix. VSS needs a VERY short period to snap the disk/ create a shadow and it cant get it. There is way more disk IO than the server can handle. 

Perfmon and certain disk counters should be your next stop

First result is a TN from Quest, that is what I was talking about

VSS Flush I/O can be common occurrence with Microsoft's Volume Shadow Copy service. It is not an issue with the Rapid Recovery agent but, since the agent depends on VSS, it is important to help address it. Note that, if you do a general search on the topic on the web, you will find that popular backup applications and their support sites address it.

The common root cause is almost always high disk i/o at the time when the agent engages Microsoft Volume Shadow Copy service. We have a KB article that goes over the possible root causes and resolution steps. We have seen that clients just resort to rebooting the machine being backed up to resolve the issue, BUT it should be investigated, and steps taken, as guided by our KB included below.

- https://support.quest.com/kb/117428/vss-backup-fails-with-the-system-was-unable-to-flush-i-o-writes-

If you take the steps suggested, and still have issues, we can always help with a closer examination if you open a service request.

I hope the above helps.