Since I was using Ubuntu, it seems that terminal cannot be opened with a shortcut such as Ctrl + Alt + t.
Click [Applications] → [System Tools] → [Settings] → [Devices] → [Keyboard] and click + at the bottom.
To add a shortcut
Name: terminal (anything is fine)
Shortcut: Ctrl + Alt + t (if you like)
Press the add button
Reference url https://sironekosukaru.hatenablog.com/entry/2018/01/20/160510