DIAGNOSIS You're missing a tiny but critical part: the plastic chute retention clip. It's a separate piece that comes in the box with , not pre-installed. Without it, the chute has nothing to hook onto.
FIX Dig through all the packaging and the cardboard box. Find the small, clear plastic bag that contains a few extra bolts, maybe a wheel nut, and a small, black, L-shaped plastic hook. That's the clip. It snaps into a specific slot on the right side of the mower deck, near the chute opening. Once it's snapped in place, the chute will hook onto it and stay upright for bagging or mulching.
IF THAT DIDN'T WORK If you truly lost the clip, you can contact Ryobi for a replacement part. In a pinch, some people use a heavy-duty zip-tie looped through the hole in the chute and around the handle, but that's a temporary hack.
HEADS UP That chute must be either fully attached (for side discharge) or fully removed (for bagging/mulching). Never run the mower with the chute just dangling loose—it's a safety hazard and will get chewed up by thrown debris.