You can’t pick up the composter bucket, just interact with it when you have collected weeds. Then it will process the weeds and spit out the red fertilizer packets that can be picked up and applied to the pumpkin.
I think the other issue you might be having is that you can only interact with things (shoo pests, pull weeds, dump collected weeds into composter) while you are not holding an item, so try dropping the watering can first (Z or J) and then interacting (X or K).
Hope that helps! Please let me know if you’re still having trouble. The interaction targets are pretty large, so I’m hoping the issue was just the empty hand requirement.