Think you are passing al the json into the template field of the Call REST Request tab? Try to add a parameter with the json i provided (with single quiotes around it). Then add the parameter to the template field. Like this:
You can do that just the normal way you would do this in a variable or any place else. See my example below. Make sure you have an object or variable with the value you want to use.
Maybe it has something to do with your JSON? Why all the double {{ and }}? Shouldn't your json look something like
{
"dtls1": {
"totalOneObligor": "10",
"flatObligo": "22"
},
"dtls2": {
"partyId": 35,
"Name": "ROAV",
"TypeId": "Personme",
"firstCreditDate": "6-MAR-18",
"lastCreditDate": "12-MAR-18"
},
"dtls3": {
"userId": "12345",
"userRole": "OFFICER"
}
}