Unity Tweak Tool Schema Missing com.canonical.desktop.interface

I had the same issue, and I used the above solutions. Both were required to achieve a solution.

sudo apt-get install notify-osd
sudo apt-get install overlay-scrollbar

Now it works perfectly.

Used

sudo apt-get install overlay-scrollbar

Works now.

For what it's worth, I had this issue on 18.04 and it was solved only by running

sudo apt install notify-osd

I didn't need to install overlay-scrollbar.

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