Hi @Vignesh,
I have tested this placeholder in my environment, and I was able to get the value inside the Subject line.
One thing to troubleshoot here would be to make sure you are running the correct report by checking the technical name (instructions here).
Please make sure that the report you are configuring the Print Management for is the same report that is actually running.
Also, did you try running the same with different Purchase orders? Does the issue happen on all of them, or only on some of them?