There is a bug report you can subscribe to:

• [keyboard panel does not allow to set up Caps as input switch][1]

The bug report reads:

Keyboard panel has options for input switching, but it does not allow
to use Caps as language switch.

Steps to reproduce:

1. gnome-control-center keyboard
2. Scroll down to ‘Typing’ section
3. Click on Switch to next input source
4. Press Caps

Expected result: Caps Lock key is accepted as switch key Actual
results: Caps key is ignored in the selection window.

P.S. Same goes for ‘Switch to previous input source’ and Shift-Caps
key combination.

Bug Workarounds

The bug is currently on the “Wishlist”. These workarounds are proposed:

Workaround 1

Found a workaround for this bug:

1. Install dconf-editor (sudo apt-get install dconf-editor)
2. Go to /org/gnome/desktop/wm/keybindings/
3. Select key switch-input-source, turn off “Use default value” and set “Custom value” to [‘Caps_Lock’]
4. (Optional) You can also change value for switch-input-source-backward to something like [‘Caps_Lock'] (again, turning off "Use default value")

Workaround 2

Another way to do this:

1. Install GNOME Tweaks (gnome-tweak-tool)
2. Open the Tweaks app
3. Switch to the Keyboard & Mouse panel
4. Click Additional Layout Options
5. Use the “Switching to another layout” section

Subscribe to bug report

I would recommend subscribing to the bug report and adding your own experiences. The more people subscribing to the bug report, the faster the solution comes (usually). [1]: https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/1709562