trying to change from pin to knock code

[AC Question]

Okay so i have an LG L22C and ive had the locl as a knock code before hut recently ive just been using the pin. But now i want to change it back but its saying its unavailable. Like the option is there but you cant click on it. Ive tried changing the wallpaper and other things but nothing seems to be working. Its also a straight talk phone if that helps at all