Preheat oven to 400 degrees. Spray half-sized baking sheet (13 x 18 x 1 inch) with cooking spray.
Using a cutting board and knife, chop the peppers into 1" pieces, the onions into 1/2" pieces, and the chicken into 1" pieces. Add the chicken, peppers, onions, and pineapple onto the prepared sheet pan.
In a small bowl, mix together the the salt, pepper, garlic powder, paprika, and Tajin. Sprinkle the spices over the chicken and vegetables, and toss everything together to evenly distribute the spices.
Bake the mixture for 12 minutes. Remove the sheet pan from the oven, and add the barbecue sauce and toss everything to evenly distribute the sauce. Return the sheet pan to the oven, and continue to bake until the chicken is fully cooked through (about 10 minutes).