Somehow I can't use ⌘ + K to clear terminal even though the shortcut has set up as default.
Any advice is greatly appreciated.
The problem is solved
The command was overwritten by an extension.
To overwrite it to reset, added the following lines in keybindings.json.
{ "key": "cmd+k", "command": "workbench.action.terminal.clear", "when": "terminalFocus" }