Note: This is not a duplicate of
MC-236767 because that bug report is about axolotls having 1.17 spawn rules that make them rarely spawn.
In 21w40a axolotls aren't able to spawn at all just like in
MC-218701, this is probably caused by something in the spawning changes introduced in 21w40a: "Axolotls now only spawn in water, above clay blocks, in lush caves"
Axolotls would spawn in water in lush caves in water
Axsolotls aren't able to spawn at all
Create a single biome lush caves world
Fly around the caves
Try to use any command that would affect @e[type=minecraft:axolotl]
You will find that it will respond with "No entity was found" every time