Please be informed that you can apply for a cashback only if your trading account is not attached to any Partner. If your account is attached to a Partner (IB), you can find a notification containing this information right on the cashback promo page:
To apply for cashback, you need to detach your account from an IB first.
Along with this, the cashback option is not available in your Quick Start bonus is active. You would need to finish the bonus steps and transfer all the bonus profits to your trading account to activate the promotion: