In a medium saucepan over medium heat, saute garlic in oil until just fragrant. Whisk in flour, seasonings, and sugar until smooth. Gradually whisk in tomato sauce until sauce is smooth.
Bring enchilada sauce to a simmer, then remove from heat and season with salt and pepper to taste. Use enchilada sauce as directed in recipe, store in refrigerator up to 1 week, or freeze up to 2 months.