Minecraft execute detect command not working

It will succesfully run the command, but when you check the commandblock you aren't standing above a wool block, so it says '[21:28:27] Failed to execute 'detect' as Heroblind55246'. Try to do something like this:

/execute @a ~ ~ ~ detect ~ ~-4 ~ minecraft:wool 15 say hi

I hope it will work!

I believe the issue here is with the damage value on the wool, or the position. The damage value of 15 is black wool, not green if that's what you were going for. Also, -4 means 3 blocks below the player, not 4 as block 0 is inside the player's feet. I ran this command with black wool 3 blocks below and it worked fine, while if there was no wool beneath me it gave me the same error.