Are you a moderator or an admin? Does directly visiting /c/slug/1234/edit work?
Edit: No, it doesn’t. The category edit dialog is specified entirely by the slug.
@pmusaraj this route can’t work on sites that have fully-stripped-ascii or “none” slug generation; you need to include the ID https://github.com/discourse/discourse/commit/6f5d8cad5105b0827ca44fd6298800384e6f40f5#diff-959bc9abc46a55332bb64d5155a79323afa75a50ec1a2137ddd22d926f62c6c5R685