when using dy in a command block, the command does not work

Please tell us what the exact command is that doesn't work. Just saying "dy doesn't work" won't help us to find out what your issue actually is.

Also, it would be nice if you could link to those issues that duplicate each other. This should not be possible.

The command is

/execute at @a[y=63,dy=100] run fill ~10 ~5 ~10 ~-10 ~-2 ~-10 minecraft:coarse_dirt replace minecraft:grass_block

I have put up two screenshots that are attached to the original post, one of the command block trying to execute it, and I executing it

The two issues that 'duplicated each other' are: https://bugs.mojang.com/browse/MC-97268 & https://bugs.mojang.com/browse/MC-79972

If you don't include dx and dz, but want to use dy, the game will just assume that dx and dz are 0. This is what MC-29972 is about.

MC-97268 is a duplicate of MC-79972, and that one is resolved as "Works as Intended", not as a duplicate.

The problem I am facing is that I want to target any player that is above y=63, not matter the x or z value.. if I put in the argument selectors dx=0,dz=0, will it work?

If you want to use the X and Z of the player, do this:

/execute as @a at @s if entity @s[y=63,dy=100] run ...

Oh ok, thanks