I tried using this command:
/execute @p ~ ~ ~ /execute @e[type=!Player,r=20] ~ ~ ~ /fill ~1 ~1 ~1 ~-1 ~ ~-1 sand
And it said that it failed to execute the above command as "Username"

What I thought would happen:
I thought that when the command block was activated, all the mobs around me would be covered in sand.

What really happened:
What I said Above^

How to Reproduce:
Put in the above command.