Stuck at "Waiting for vehicle to initialise" when using DroneKit 2.9.2 and APM 2.6 with Firmware 3.2.1

Question

I'm working on a project using a drone equipped with an APM 2.6 flight controller running Firmware 3.2.1. I'm trying to establish a connection and perform a simple mission using DroneKit version 2.9.2 and MAVProxy version 1.8.67 on Mission Planner version 1.3.36.

Here's the Python code I'm using:

from dronekit import connect, VehicleMode, LocationGlobalRelative
import time

# -- Connect to the vehicle
import argparse
parser = argparse.ArgumentParser(description='commands')
parser.add_argument('--connect')
args = parser.parse_args()

connection_string = args.connect

print("Connection to the vehicle on %s" % connection_string)
vehicle = connect(connection_string, wait_ready=True, timeout=60, baud=57600)

# ... (rest of your code)

When I run this code, I encounter the following output in the terminal:

CRITICAL:autopilot:Calibrating barometer
CRITICAL:autopilot:barometer calibration complete
CRITICAL:autopilot:Initialising APM...
 Waiting for vehicle to initialise...
 Waiting for vehicle to initialise...
 Waiting for vehicle to initialise...
 ...

And it doesn't proceed beyond "Waiting for vehicle to initialise." I have already checked the hardware connections, calibrated sensors, and confirmed that my drone's battery voltage is sufficient.

Could someone please provide guidance on how to troubleshoot and resolve this issue? Any insights or suggestions would be greatly appreciated.

Thank you in advance!