I installed Wireshark but it didn't ask me whether I wanted to give it permission to capture packets. And when i entered sudo usermod -a -G wireshark (username) into the terminal, I got an error telling me that the wireshark user group didn't exist. Do I have to manually create the wireshark user group, and if so, how?

Look at the following links.
https://askubuntu.com/questions/74059/h ... 4064#74064
https://wiki.wireshark.org/CaptureSetup ... Privileges

sudo groupadd wireshark
sudo usermod -a -G wireshark $USER
sudo chgrp wireshark /usr/bin/dumpcap
sudo chmod o-rx /usr/bin/dumpcap
sudo setcap 'CAP_NET_RAW+eip CAP_NET_ADMIN+eip' /usr/bin/dumpcap
sudo getcap /usr/bin/dumpcap
Then log-out and log-in again.