[2020] 2.7.2 - Can't play with the Ancient Relics DLC

That's an odd situation, you'd think if that was a bug we'd have more reports of this.

Can you try doing a test installation on any other machine? Just to see if it's a problem on this machine or somethign else.  

BladeRnr said: Can you forward this to the dev team so they can check it out?

Actually you can do that yourself! Right here:

New launcher version : 2020.13

Slightly delayed release, matched by a slightly delayed announcement. I'm all for symmetry in life. In any case, the latest version of the launcher brings us to a natural rest-stop when it comes to playsets, and we'll be persuing some other... forum.paradoxplaza.com
However if this is a launcher bug, why would it only affect you? Wrong checksums are more commonly a corrupt installation or download problem. Did you want to try a clean reinstall of Steam, game and launcher all together? Let me know if so and I'll devise a procedure for that.