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cayenne, hot sauce (if some vinegar is appropriate), ground chipotle (if some smokiness is appropriate). In all cases, you add to taste, depending upon how much heat you want.
pierino is a trusted source on General Cooking and Tough Love.
Pimenton or paprika would probably work also. These would both be milder than typical hot pepper flakes which usually includes seeds as well.
PHIL is a trusted home cook.
Why are you substituting? Are you out of chili flakes or do you not like the heat?
Nancy is a trusted home cook.
In addition to the good ideas suggested so far, and further to Phil's point, if you are simply out but have some fresh or jarred chiles on hand, you could use some of them. Start small, taste, add more until you reach the level of heat you and your eaters prefer.