I’ve just taken another look at this and it seems like you’re trying to set the PWM of servo 2, which is pretty much always assigned as a motor for use in controlling the vehicle (which generally need to be controlled with MANUAL_CONTROL or RC_CHANNELS_OVERRIDE messages, while the vehicle is armed).
If you want to use the DO_SET_SERVO command you’ll need to ensure the SERVOn_FUNCTION parameter (where n is the servo you’re trying to control) is set to Disabled (-1). This is not recommended for motion control outputs, so normally you would only do this for outputs 7+ in a standard BlueROV2, or 9+ if you’re using the heavy configuration.
If you’re using a Pixhawk note that servos 1-8 are MAIN 1-8, and servos 9-14 are AUX 1-6.