I think you undo that screw at the end of the bar, remove the stopper, slide the handle back on, then replace the stopper.
Looking at it, my take on this (and I know nothing about Ortlieb bags) is that you should push the clip over the skinny bit of the bar in the dead centre, and then slide it to the left. Having probably given everything a squirt of WD40 first.
That should be easy enough to try, and doesn't involve undoing anything, so worth a bash just to eliminate the possibility.
View attachment 752399