It was frustrating as hell and if you are facing the same issue, I can understand your frustration.
Let me show you the quick and easy fix for this annoying issue on Ubuntu.
Fixing drag and drop issue on Ubuntu 18.04
The reason behind the drag and drop not working was the experimental view of GNOME Files (previously known as Nautilus).
I don’t remember if it was activated by default or if I enabled it manually while trying some Nautilus hacks. But this experimental view resulted into drag and drop not working. I guess that’s the ‘side effect’ of too much tweaking. Curiosity killed the cat and in my case, it killed the drag ?
To fix this, all you have to do is to disable the experimental view again.
Open Nautilus file manager. On the top left, click on Files->Preferences. Clear the “Use the new views” option under Experimental.
Fix drag and drop not working in Ubuntu
Close the file manager and open it again. Your drag and drop should be working again.
Alternatively, if you prefer the command line, you can use gsettings to fix this annoyance by using this command in the terminal: