I have reputation 1, if that matters. Yesterday I asked a question, and I was advised to create a room for this question as it was too broad, so I deleted it. I don't know if this matters.
The title of your post says you can't create a chatroom on Software Engineering, which would be correct.
Seeing that you can't create a new chatroom unless you earn 100 reputation. This is documented in the /privileges page of each site. The link can be found in the top right corner of your achievements.
To participate in chat you need 20 reputation.
Also when creating a chatroom it is not always obvious that you are creating it on a different Stack Exchange site. If you opt to create a chatroom it will default to creating it on Stack Overflow (in my case). You can select the community you want it associated with as per screenshot: