Hi @chang-M,
Operating a vehicle generally involves more than just commanding it to move, and you haven’t provided code or described the steps you’re taking to set up the connection to the autopilot.
There are some failsafes that may be relevant, but more generally the HEARTBEAT, SYS_STATUS, and STATUS_TEXT messages may help you to determine if/when something is going wrong, and what is causing it.
Have you tried controlling the vehicle with a standard control station software like Cockpit or QGroundControl, to see whether that also has issues when you try to enter depth hold mode? If they have the same issue then there might be a problem with your external pressure sensor (e.g. it may be disconnected, or damaged), which may be preventing the autopilot from estimating its depth. In that case you may wish to submit a product problem report, to get help from our support team, or to wherever you bought your depth sensor from.
If it works fine with a standard software then the issue is most likely with your code, in which case I’d recommend posting your code and describing the difference in observed behaviour from what you’re trying to get it to do.