Issue resolved by Mathworks support team. This is the solution,
This issue appears to be caused by a small syntax error in the code within the "px4_msgs" folder.
As a workaround, could you please do the following:
1. In the MATLAB command window, unzip the "px4_msgs" folder:
unzip("px4_msgs.zip")
2. Navigate to px4_msgs/msg/VehicleCommand.msg file, and open it
3. Scroll to the end of line numbers 40 and 41 in VehicleCommand.msg, and remove the "*/" symbols from both lines
4. Run "ros2genmsg("custom")" again
This worked for me.
Thanks