Javascript uncaught type error is not a function Builder Pattern

I am building a validator using a Builder Pattern. Not sure if I am using the correct pattern but I keep on having Javascript uncaught type error is not a function although my codes works.

Full Error msg:

Uncaught TypeError: validate.IsTCAgree(...).IsRMGTCAgree is not a function 
at HTMLButtonElement.<anonymous> (TPR:1846:115) 
at HTMLButtonElement.dispatch (jquery-3.5.1.js:5429:27) 
at HTMLButtonElement.elemData.handle (jquery-3.5.1.js:5233:28)

$btnSubmit.click(function(e) {
      var validate = new ValidatorTnCBuilder($btnSubmitErrMsg);
      if (validate.IsTCAgree('Please check Terms and Conditions.').IsRMGTCAgree('Please check 
          agreement.
          ')) {
          // do sth
        }
      });
    const ValidatorTnCBuilder = function($el) {
      return {
        IsTCAgree: function(msg) {
          if (!$checkIsTCAgree.is(':checked')) {
            $el.text(msg);
            return false;
          }
          return this;
        },
        IsRMGTCAgree: function(msg) {
          if (!$checkIsRMGTCAgree.is(':checked')) {
            $el.text(msg);
            return false;
          }
          return this;
        }
      }
    }