Spray cold grates of grill with grilling spray and heat to medium. Season chicken with salt and pepper.
Grill chicken until charred on both sides and no longer pink in center (165°F). Place chicken on a plate and cover tightly with foil.
While chicken rests, cook fajita vegetables according to package directions, until sauce is thickened. Slice chicken into thin, bite-sized pieces. Serve chicken and fajita vegetables in warm tortillas.