I lost my top panel icons after upgrading from 18.04 to 20.04. I had some useful apps like the calculator and terminal and I wanted their icons always visible. Many tutorials suggested right clicking on the panels to modify or dragging icons onto the panels but neither option worked for me. When I dragged, it opened the application instead of moving the launcher. When I right clicked, I did not see any "Add to Panel" or other options for making changes - all that was in the menu was Help and About Panels.
I determined that the panels were locked, and could be unlocked via the dconf editor. I opened
dconf-editor and went to
org.mate.panel.general and set
locked-down to "0". I was then able to drag and drop my applications to the top panel.
Note: I had the Traditional Panel layout selected while doing this.