How can I empty the trash using terminal?

You can use the rm command:

rm -rf ~/.local/share/Trash/*

The rm command removes (deletes) files or directories.

-f, --force     Ignore nonexistant files, and never prompt before removing.
-r, -R, --recursive     Remove directories and their contents recursively.

The trash folder is found at: $HOME/.local/share/Trash

Be careful how you use the rm command - the files aren't sent to a trash can where you can undelete them, so it's not easy to undo.

After you sudo apt install trash-cli, you can do

trash-empty

More interesting details about trash handling below and in the man page.

restore-trash (1)    - Restore for Command line trash utility.
trash (1)            - Command line trash utility.
trash-empty (1)      - Empty for Command line trash utility.
trash-list (1)       - List trashed files.
trash-put (1)        - Command line trash utility.
trash-rm (1)         - Removes files matching a pattern from the trash can

You are looking for the $XDG_DATA_HOME/Trash directory. The trash directory is defined in the "Desktop Trash Can Specification" of the freedesktop site. This variable is normally not available in the terminal windows, hence you will need for trash-empty. This command follows all the specification of the Freedesktop.org and it's intelligent enough to find out where the Trash is. You will need to install it first.

There are other tools for this, like gvfs-trash --empty which can also send items to the Trash can.