Users new to Linux/Unix are frequently confused when they run a command as indicated in the WeeWX documentation and get a 'permission denied' message.

pi@pi4p1:~ $ weectl extension install /var/tmp/gw1000-0.3.1.tar.gz
Request to install '/var/tmp/gw1000-0.3.1.tar.gz'
Extracting from tar archive /var/tmp/gw1000-0.3.1.tar.gz
Traceback (most recent call last):
   [...]
PermissionError: [Errno 13] Permission denied: '/etc/weewx/bin/user/gw1000.py'

The problem here is that weewx installs things as the privileged user 'root', yet the user is using a non-privileged user 'pi' to try to do privileged things.

Preface the command with 'sudo' so that your command runs with elevated privileges.

pi@pi4p1:~ $ sudo weectl extension install /var/tmp/gw1000-0.3.1.tar.gz

For more about permissions, see the the wiki article Understanding permissions.