SAP C4C + SDK + Unable to update KUT value from SDK code. Getting error its read only

Hi Experts,

I have created KUT field with text 40. From C4C UI , I can update the value and save it.

For my requirement i am trying to update KUT Value from SDK in script ( validation -onsave).

I have the reference hence i can read the file but while updating i am getting error that this KUT field is read only.

Code in the script is very simple

this.kut_field = "updated field value" .

error screen shot

error-screen-shot.jpg

Thanks