REST API update error

I am using vuejs with SharePoint. I get the following error in the function below.

"The property 'columnName' does not exist on type 'SP.Data.RegisterListItem'. Make sure to only use property names that are defined by the type."

Here's my internal name:

I am passing in 'RiskRegister' as an argument to updateField. How will I use the parameter columnName, which represents 'RiskRegister', in my function?

HTML Side <input type="text" v-on:change="updateField((risk, 'RiskRegister', risk.RiskRegister))" v-bind:data-id="risk.id" :readonly="!risk.editable" v-model="risk.RiskRegister">

JavaScript (Excerpt)

 function updateField(risk, columnName, columnValue){

        var item = {
          "__metadata": { "type": 'SP.Data.RegisterListItem' },
          columnName : columnValue
        };

       $.ajax({
          url:  _spPageContextInfo.webAbsoluteUrl  + "/_api/web/lists/getbytitle('Register')/items(" + risk.Id + ")", ///_api/web/lists/getbytitle('register')/listitementitytypefullname
          type: "POST",
          contentType: "application/json;odata=verbose",
          data: JSON.stringify(item),
          headers: {
              "Accept": "application/json;odata=verbose",
              "X-RequestDigest": $("#__REQUESTDIGEST").val(),
              "IF-MATCH": "*",
              "X-HTTP-Method":"MERGE",
           },
          success: function (data) {
               alert("success");
          },
          error: function (data) {
               alert("failed");
          }
        });

}

Any help would be much appreciated.

Thanks!