If you can't open the game to change settings, you have to do it from the outside. Locate the File : Go to your Steam folder, usually:
Before diving into fixes, it helps to understand the enemy. The Serious Engine 3.5 (which powers this game) handles display initialization poorly in modern environments. The error appears when: serious sam 3 cannot set display mode
If the quick checklist doesn’t help, proceed with the full steps below. If you can't open the game to change
Modern Windows features can interfere with legacy fullscreen modes. usually: Before diving into fixes