When trying to fill air blocks generated in the world in order to generate caves, ravines, lakes, etc. are not counted when using /fill <x> <y> <z> <x> <y> <z> <block> replace minecraft:air. Once a block has been placed in that position and then broken, it will be properly filled. Otherwise, the command will return the error message "No blocks were filled."
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yeah underground structures are actually filled with cave_air
you need to use both /fill <x> <y> <z> <x> <y> <z> <block> replace minecraft:air, /fill <x> <y> <z> <x> <y> <z> <block> replace minecraft:cave_air at a time or use /fill <x> <y> <z> <x> <y> <z> <block> keep

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We're tracking this issue as MC-125135, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as Won't Fix, which means this is considered a bug but won't be fixed. The description of that ticket or the comments might explain the rationale. Please do not leave a comment on the linked ticket.
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You need to use "cave_air" https://minecraft.gamepedia.com/Air