We found that the XRCrossTabCell.Text expression cannot be edited in End-User Report Designer when the office-inspired Property Grid is used. I forwarded this issue to our developer team for further research. We will post an update to this thread once we have any progress on this task.
As a temporary workaround, you can switch the Property Grid to a classic view, as described in the Switch to the Classic View help topic. To change an expression, select the desired XRCrossTabCell element and use the steps demonstrated on the image below to modify/create an expression: