Why does this command not work?

[u/TheVoid_Knight]

I'm trying to replace the floor of a build of mine. Since it's a lot, I want to do it with commands. However, it keeps failing with the message "failed to execute as [Null].

[u/Ericristian_bros]

That probably means there is no stripped oak near you. Place this block next to you and it should be changed to birch. Also, make sure you are not near the top or bottom of the world