15 days ago - AndrewT - Direct link
When you say the launcher, do you mean as run from the Steam play button? Or are you running it in the standalone mode , either from a desktop shortcut or its .exe file directly?  
14 days ago - AndrewT - Direct link
The point is that purchases from the Steam store are not supported in the launcher in standalone mode, it's only intended for purchases from our store. So please only run the launcher/game from Steam.

If this DLC show sin your Steam "Manage my DLCs" page as enabled - does it? - then it absolutely ought to be in our launcher's DLC tab too. Frankly hard to see why it would not; I see it in your account here as having synced in from Steam for instance.

Can you please try a test install of Steam/this game on any other machine you might have access to?  
13 days ago - AndrewT - Direct link
Please do a full **clean** re-install of Steam and game:

- move any valued save games elsewhere, and user_empire_designs.txt from /Documents/Paradox Interactive/Stellaris/ if you have made any custom empires or races.
- "uninstall" in Steam-Stellaris
- exit the Steam app entirely

- manually delete both the Steam/SteamApps/common/Stellaris AND Documents/Paradox Interactive/Stellaris folders
- delete everything in the Steam folder EXCEPT steam.exe and the SteamApps folder
- run steam.exe to rebuild all that

- re-install game in Steam, run a Steam Verify when done.
- add stellaris.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
- start the game with no mods active and test

Good luck!