I'm trying to make a discord bot/webhook in a module script so I can execute it whenever I want. If I use it, all it does is error: HTTP 400 (HTTP/1.1 400 Bad Request)
I have been having a hard time with this and have been searching for an hour and a half with no answers for roblox scripting.
The answer to your question is "It can". You see, a ModuleScript can be run from the server or the client. If you require the ModuleScript from a Script, it is running on the server and so will be fine (since you can only use HttpService from the server). Similarly, it won't work if it's a LocalScript that requires it.
Since you're getting that error, I believe this confirms that you are referencing HttpService correctly and from the right place - the problem is that you're using it incorrectly (sending in a bad http request).
See here for details on HttpService; it also has links on how to properly send requests.