One of the most annoying things about Ubuntu (and Linux in general) is that USB serial ports can only be accessed in superuser mode unless something is done. The result is that you can see the port as /dev/ttyXXXX but can’t open it. While there are other ways, adding your user name to the dialout group will usually fix this:
sudo adduser <username> dialout
Then log out and log in again to kick in the new setting.