Command Block Bug

Did you renewed the power to the command block (eg. destroying and replacing the redstone block.)? If you didn't, the command block won't re-execute the command.

Also notice that the gamerule DocommandBlockOutput is for the output in the chat only, and not for the output in the command block itself. The window of the output in the command block is only empty because the command block didn't execute its command yet.

In the first screenshot, you can see that I haven't placed the Redstone block, but in the 2 you can see that I have placed it.

(and I know that it only shows in the chat, but it just doesn't show it)

You know you can type commands in chat, right?

Why should I use commands in chat if I need to have them permanently for an Adventure map

You might need to enable the sendCommandFeedback gamerule.

fixed it