I had the same problem! The liner had two additional fixings that weren't mentioned in the guide. One was in roughly the position you're looking at but on the other side of the metal from the one you've photographed.

If you follow the liner with your fingers, you will feel it held in place with a 10mm bolt on the underside of the car. It's very fiddly and smaller hands are more effective. It's impossible to get a tool to the bolt without first removing the middle under-tray