Looks like you're on Fedora Silverblue (or other Atomic version). This is happening because the system groups are in /usr/lib/group rather than /etc/group and this causes the issue you're seeing here. You can work around it by getting into a root shell with something like
sudo -i
and then getting the group added to /etc/group with
grep -E '^dialout' /usr/lib/group >> /etc/group
after that, you'll be able to add your user to the group with
/etc is writable, so no reboots are required. That said, /etc is treated in a special way and each deployment will have its own /etc, based on the previous one.
So if you make changes to /etc then revert to a previous deployment, your changes will be reverted as well. But if you make changes and upgrade (or do whatever to create a new deployment), your changes will bu preserved.
It's like when I run into some issue with how I've set up my system in NixOS and have to explain to a non-Linux user that it isn't Linux that's the issue but how I'm using an especially weird Linux lol
Yeah this one is ridiculous. There are some systems that have bounced my password ... literally the one stored in a password manager ... and gaslite me that I "must have forgotten my password."
Even if "isn't that bad" were true, it's hardly a stunning endorsement. I wish Linux aimed higher than "not that bad", but it always seems to hit "only some bits are broken".