Gimbals can work with either one or two IMU sensors. One sensor is placed above the camera to track camera positions, and the second (optional) is typically mounted to the vertical frame post for tracking frame angles.
If you are using a Dual IMU system and cannot click on the Frame IMU button, it's possible this option has been disabled. Click over to the Advanced Tab and change the option for Frame IMU to Below YAW. Next Click the Write button to save your changes.
Now return to the Basic Tab and try to select the Frame IMU button for your options.