in 1.16.5, l try to search these mob Frostmaw , Grottol. l only found mob is Foliaath, Ferrous Wroughtnaut, Barakoa, Barako, the Sun Chief. l setting easy to hard but not found one of these Frostmaw , Grottol. l am not sure is my error because l using a lot of modded ,l don't know all my modded effect Mowzie's Mobs ? that is error mob not spawning in biome when selected easy to hard

l see thanks you, but l turn off command in minecraft :)) that mean l can't use command

Difficulty doesn’t matter. You are likely searching in the wrong places.
You can use a /locate command to find frostmaws. Grottols are very rare underground at diamond levels. Lanterns are only in roofed forests by default, and nagas are only in stone shores.

You can check in a new creative mode world to confirm that they are generating.