The /dev directory is recreated at every boot, so your chmod vanishes.
The better way is to determine the group ownership of the device withls -l, then add yourself to that group with sudo adduser $USER group
Here's my expanded explanation:
Open a "terminal window" by pressing Ctrl-Alt-T.
Left Click to select this window.
Explore. Type:
ls -l /dev/ttyACM0
man ls
Fix. Type;
sudo adduser $USER $(stat --format="%G" /dev/ttyACM0 )
When prompted, type your login password.
This will set things up so that your next (and all subsequent) login will have group access to /dev/ttyACM0.
Therefore, logoff, login and be happy.