Cannot add applications to Plank

[wheattobread]

Hello everybody,
I've tried to use plank many times, but it was unuseable for me. The main reason is that plank does not allow me to add applications by dragging (even files in usr/share/applications don't work) nor does it allow me to keep an already opened applications in dock. Furthermore, it randomly pick apps to place in the dock (doesn't even sync with my panel). I'm using linux mint 20 cinnamon. Can anybody advise me on what to do if I want to use plank? (btw I tried to install it using both the terminal and the software center and it still failed. I've read and tried many ways posed on the internet but none of them worked)

[LoVache]

Hi, launch an app: it comes in your dock, then make a right-click on its icon and check "Pin to dock". Finally grab this icon to place it where you need in the dock.
(translated from French)
To find Preferences: Ctrl + Right-Click on the dock.
https://launchpad.net/plank