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!

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!