Have you tried using warm wet compresses on the preen gland? If it's blocked that may loosen it up and get it unblocked. I would do those and very gentle massage to see if you can get it unblocked. If it doesn't resolve then a vet visit may be best, if it's infected then antibiotics might be needed.
https://www.beautyofbirds.com/birdoilgland.html
https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/uropygial-gland
https://www.beautyofbirds.com/birdoilgland.html
https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/uropygial-gland