Custom buttons not working in zoho recruit

Custom buttons not working in zoho recruit

I am trying to add a custom button  in zoho recruit and I created a button with existing function in gallery (Screening candidates). When I try to run the function by clicking on the button I didn't get any response (even though I haven't done any changes to the existing function). I tried with and without providing my authtoken. When I check in the console, I saw this error. 

Uncaught SyntaxError: Unexpected token < in JSON at position 0
    at JSON.parse (<anonymous>)
    at Function.jQuery.parseJSON (zcrm-libs.js:39)
    at Function.e.parseJSON (zcrm-libs.js:40)
    at zcrm4.js:58
    at Object.success [as successfn] (zcrm-libs.js:35)
    at Object.pushPendingResponse (zcrm-libs.js:35)
    at Object.processResponse (zcrm-libs.js:35)
    at Object.success (zcrm-libs.js:35)
    at fire (zcrm-libs.js:37)
    at Object.fireWith [as resolveWith] (zcrm-libs.js:37)