Hi Maru,
Here’s a couple things to try/check out.
In that image it looks like there is already a driver on that shapekey. If the driver isn’t correct, try deleting it first.
It also says “Active Shape Key Index”. To me that means it’s actually the shape key that is highlighted (or active) and not the actual value that you are trying to paste the driver to.
Try clicking on the number portion of the shape rather than the name portion.
