I’m new to ArduPilot, so I’m still learning.
I’m creating an unmanned catamaran using PIxhawk 6C and the Rover setup as instructed on site.
I can remote control it perfectly. But when I send it on a mission, it navigates backwards and spiraling to outside.
You have likely reversed RC channels instead of the servo output channels. Set the RC channels back to default, and configure the servo channels to do the reversing as required.
Follow the documentation carefully. If you are selecting auto mode before tuning (which would reveal this issue, which is avoided via proper setup in the first place), you are skipping many steps.