Server commands other than /me, /announce are blocked

That means you can only use /me or /announce, due to security reasons.

This is why your command starting with a slash confuses Nightbot—it sees an unauthorized slash command (anything other than /me or /announce).

To fix the issue, either remove the slash from your command, or insert an invisible character (one Twitch doesn't auto-remove, unlike a space or separator) before the slash at the start of your command response.

I don't have any commands and I don't have a invisible character.

The (.) serves the same function as / in Twitch IRC chat, so if you remove this, you'll get the command's response.