Please re-read the top of
http://weewx.com/docs/debian.htm
There are 'four' commands you need to run.
Under 'configure apt':
* run the command under 'Tell your system to trust
weewx.com'
* run the command under 'For Debian10 and later, use python3'
Under 'Install':
* run the first command there
* run the second command there
You didn't run all four commands.