Step 1:
Combine flour, garlic powder, salt, and black pepper in a shallow dish. Step 2:
Roll and coat chicken cubes in flour mixture. Step 4:
Heat 2 tablespoons vegetable oil in a skillet over medium-high heat. Step 6:
Cook and stir chicken in hot oil until not longer pink in the center and juices run clear, 8 to 10 minutes; remove and set aside. Step 8:
Heat 1 tablespoon vegetable oil in the same skillet over medium heat. Step 10:
Cook and stir celery, green peppers, and onion in heated oil until slightly tender, about 5 minutes. Step 12:
Return chicken to the skillet. Step 14:
Whisk ketchup, lemon juice, pineapple, and brown sugar in a bowl; pour into the skillet; bring to a boil. Step 16:
Cook and stir chicken and vegetables in sauce until heated through, 2 to 3 minutes.