/dev/ttyACM0 should work on linux for talking to the core over serial. I usually do something like “minicom -D /dev/ttyACM0 -b 9600”
It sounds like you were able to connect and hit “i” to have the core give you its id if you entered it on the website? It’s also important that your core be online when you try to claim it. Can you confirm it’s connected to your wifi and breathing cyan? Did the “claim a core” prompt give you any errors? If not, and you’re on Kubuntu, what browser / version are you using?
If it’s breathing cyan and you have the core id, can you email me (firstname.lastname@example.org / email@example.com) your core id, and the email address you’re using on your account if you have it?