Preheat the oven to 425°F and line a baking sheet with parchment paper.
Slice the chicken breast into bite-sized pieces and pat dry with paper towels.
In a shallow bowl, add the coconut flour. In a separate small bowl, whisk the egg white until frothy.
Dip each chicken piece into the egg white, then dredge evenly in the coconut flour to coat lightly.
Place the coated chicken pieces on the prepared baking sheet, ensuring they are spaced out for even cooking.
Bake the chicken for 20-25 minutes, turning halfway through, until golden and crispy.
While the chicken bakes, prepare the sweet and sour sauce by combining pineapple chunks, bell pepper, rice vinegar, and soy sauce in a small saucepan.
Heat the sauce over low-medium heat, stirring occasionally until the ingredients meld together and warm through.
Once the chicken is done, transfer to a serving plate and drizzle the warm sweet and sour sauce over the top.
Serve immediately and enjoy your delicious, protein-packed crispy baked sweet and sour chicken.