Google the writing on the capless van filler point: Easy Fuel
There should be a special plastic filler adapter somewhere in the vehicle. My Galaxy it was behind a panel / trim inside the rear near the filler. Spare wheel well is likely another favourite spot. The petrol and diesel versions will be slightly different designs.
Your diesel filler neck has a device fitted to help prevent you filling it with petrol. The nozzles on petrol guns are physically smaller than diesel and cannot easily press on the two little fingers on opposite sides of the hole at the same time. This is what is required to open the filler for diesel
For more talking and walking through, see this video for example:
If you’re still struggling, try cutting the funnel nozzle end off (eg where your thumb is in the picture) so it’s no longer tapered and see if the larger profile it now has is sufficient to press the fingers.
Why are you filling from a can? If you've run out of diesel, that’s the least of your problems as it will require bleeding. If not, I’m assuming you’re buying knocked off fuel.
You might find a white funnel intended for the job in the glovebox or with the spare wheel/jack. A funnel would have been supplied with the vehicle when it was new.