Guild Hall, Smithy, and Museum are locked at level 2 right now. Since Guild Hall unlocks content, I've put a hard cap on the level until I'm ready to start adding tier 3. There's currently no upgrades for Smithy 3 yet either, and Museum is locked at 2 only for balance reasons right now (only slowly increasing it). In general, any building that says "Cannot currently upgrade" is either locked there for balance reasons, or that there's no content beyond that point yet.
first rebirth forced so you have learned combo system talent. you can see current combo bonus at left top