Look inside the beak for yellow plaques and a very bad odor and tell us what you find. Take a picture. Canker might be the problem, and it can cause the side if the beak to bulge out. The nostril on the left appears to be blocked or covered as well. I would try to clean that blockage out. Using QTips and a bit of hydrogen peroxide to help soften the material can help. Then use a fingernail or wooden toothpick to gently try to clean out the material, taking care not to damage the nostril. Get someone to help hold the chicken’s head still.
Canker inside the beak can look similar to wet fowl pox, or yeatsst/fungus infection. Canker smells bad. Canker is treated with certain drugs such as FishZole (metronidazole) or Ronidazole which are found on fish and pigeon sites.
Canker is very contagious, and your feeders and waterers should be cleaned out and disinfected with 10% bleach water. Prevent chickens from drinking from puddles.