requests is not a built in standard library; even on a standard Python installation, you have to manually fetch it. Also, Ignition 7.9 is using Jython 2.5, so you’ll have to find an older version of requests that works with 2.5.
Once you’ve tracked down a version that should work, you can manually add it to the user-lib/pylib folder on the gateway and it’ll be synced to clients and designers for you.
That’s requests version 2.5.0. I have no idea if it’s going to work on Python 2.5, but given the latest version is 2.26, it seems unlikely their versions are aligned with Python major versions.
Stepping back a bit - what script are you trying to execute? If it's got a dependency on requests, then it may have other dependencies that aren't met by the Jython 2.5 environment you're in. Where did this script come from and what is it supposed to do?
For simply executing http requests, there's lots of options in Ignition, even in 7.9, including the builtin system.net.httpGet, for instance.
I managed to solve the above problem,
Issue was i was referencing wrong parameter for httpGet, instead of hearders it should have been headerValues.
Now the code works just fine.