Adding login items from terminal in Catalina
Try enabling AEServer and/or Script Editor in System Preferences > Security &amp; Privacy > Accessibility. Occasionally I've had to quit and relaunch System Preferences for new login items added via AppleScript to appear.

After doing this I was able to add login items via AppleScript. Confusingly, after I disabled Accessibility access for AEServer and Script Editor I was still able to add login items via AppleScript.