With most Canon EOS cameras, second curtain sync is only available with shutter times longer than 1/30 second. However, the camera will usually allow one to set second curtain sync with a compatible flash, it just won't be used if the exposure time is shorter than the threshold.

Any manual control of flash sync is only allowed in P/Tv/Av/M exposure modes. If you are using the camera's Fully Automatic Shooting (Scene Intelligent) exposure mode the camera will insist on making those decisions for you.

Second curtain sync is excluded in Bulb exposure mode since the camera can't "anticipate" the beginning of the second curtain closing before the shutter button is released. The camera needs enough time to trigger the flash to allow for the flash duration before the second curtain begins to close. But in B mode, the second curtain begins to close the instant the signal from the shutter button (or button on the wired remote, or signal from the radio remote) is received. If the flash doesn't fire until the second curtain begins to close, the exposure will be uneven across the frame as the shutter curtain moves across the sensor (or film) while the flash releases its energy over time.

If you are in one of the P/Tv/Av/M exposure modes and you still can't select second curtain sync, the issue is probably with the current settings on the 600EX II-RT. In general, if a "non default" setting is selected using the flash's own menu, Canon EOS cameras will not override that setting. In order to control the flash settings from the camera's menu, the corresponding setting on the flash itself should be set to the default value. For example, exposure compensation should be set to "0", manual power should be set to 1/1 (full power), zoom head should be set to "Auto", etc. In the case of flash sync, the default setting in the flash's menu is "First curtain." If "second curtain" is already selected on the flash, the camera will not allow you to override that using the camera's menu.