Unable to place woodland mansion, end city and monument in default superflat world

in default world too

In default world I was able to place all 3 of them. Only flat worlds didn't work.

This is because the default superflat worlds don't have blocks at the required height (the Y position is too low to place these structures). You can in fact place mansions and end cities in superflat worlds with enough height.
Try this preset:

minecraft:bedrock,200*minecraft:dirt,minecraft:grass_block;minecraft:plains

The preset you sent me works, but I think Mojang needs to alter the default superflat setting.

the woodland mansion's files are completely unloaded on default superflat worlds. you can see this with a structure block.