I cannot access the setting in Ubuntu. Whenever I want to go to the setting by any method, Ubuntu would automatically collapse and go to the login page. After I log into the system later, the screen is just like when I first turn it on and nothing I did before is stored.
I guess it may relate to my work did before: I fell into a login loop when I installed the ubuntu. It was caused by Nvidia Graphic issues and I fixed it by doing the steps illustrated in this answer: Graphics issues after/while installing Ubuntu 16.04/16.10 with NVIDIA graphics . I just did steps 1-4 in section 3 and it works.
I am using ubuntu 18.04.