Have the extensions been loaded on server and appear correctly?
What version of Tableau Server is in use? I had issues with 2018.2 upgrading to 2018.3 and needed to restart the server to get them to work properly after the upgrade.
I’m using 2018.3, I did manage to get it working in the end.
I had to go to the developer website and find the files I needed manually then drop them in my file system locally.
The issue was that the download link on the extension gallery doesn’t work for me. I was trying to download the brush extension.