I just would like to thank you dash for sharing your experience here. Thanks to your information, I just went through the DPP course successfully! Mine was as follows.
$ pip freeze | grep -i mav
$ pip freeze | grep -i drone
As you suggested, what I did was to downgrade pymavlink==2.4.15–>>pymavlink==2.4.8 in the virtual environment and hand over vehicle object to some functions. Since I use a very old laptop (EPSON Endeavor) in which Ubuntu 20.04.2 LTS was already installed, your information helped me a lot!