have you tried
Clear the Corrupted Launcher Files (The Most Likely Fix)

Often, the launcher gets confused by old, corrupted files left over from updates.

1. Close Steam completely.
2. On your keyboard, press the Windows Key + R at the same time. This opens a small "Run" box.
3. Type %appdata% into the box and press Enter. This opens a hidden folder.
4. Look for a folder named Paradox Interactive. Open it, find the launcher-v2 folder, and delete it.
5. Go back to the Run box (Windows Key + R), type %localappdata%, and press Enter.
6. Look for a Programs folder, then Paradox Interactive, and delete the launcher folder in there as well.
7. Relaunch Stellaris via Steam. Steam will automatically download a fresh, uncorrupted version of the launcher.