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

usermod -aG dialout pipe

Enter Password: ********

The password you entered is incorrect. Would you like to reset your password?

Y

Please enter your new password: ********

New password cannot be the same as old password.

😑

If you want to add an existing user to an existing group, use:

usermod -a -G <group> <user>

I've had this one recently.

It gives you an error message, but creates the group anyway.

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".

😃