How to get more information about "Error loading extension"?

Solution 1:

Run

sudo journalctl /usr/bin/gnome-shell

and look for errors associated with the extension while trying to enable it. Alternatively you can use grep to filter logs by extension name:

sudo journalctl /usr/bin/gnome-shell | grep 'system-monitor'

Also sometimes an extension works alright if you just manually add your GNOME shell version (which is not officially supported by the extension) to the extension's metadata.json file.
To do that (if you have installed the extension not as the root) go to ~/.local/share/gnome-shell/extensions/ and look for the directory associated with the extension. Open it and you should find a metadata.json file inside. Open the file, add your GNOME shell version under the "shell-version": section and save the file. Relogin and if you're lucky the extension may start working.

Solution 2:

You can see details of GNOME extensions, including errors, using GNOME's Looking Glass tool:

1. press Alt+F2, type lg, then Return.
2. go to the "Extensions" section
3. search the relevant extension and click on "Show Errors" for it