This constant was added in Restrict Content Pro version 3.3. You must be on this version or later for it to take effect.
When set, this constant forces Restrict Content Pro to always behave in sandbox mode.
define( 'RCP_GATEWAY_SANDBOX_MODE', true );
This is a good idea for all dev/staging sites to prevent you from accidentally making changes that will affect the live gateway. For example, if your staging site is in live mode and you cancel a membership on your staging site that has a recurring Stripe subscription, then that subscription will be cancelled in your live Stripe gateway, and thus also cancel the membership on your live site. By keeping your staging site in sandbox mode you prevent this from being able to happen. Any cancellations you try to make in sandbox mode will not affect the live gateway.