Need Assistance with capturing HTTP Request and Response Details on F5 with iRule on LTM 12.X.

I may be wrong, but you can't capture source ip in the HTTP_REQUEST_SEND. So have the variable set in the HTTP_REQUEST event itself.

when HTTP_REQUEST {
    set SRCIP [IP::client_addr]
    log local0. "Source IP address is $SRCIP"
    }