Sometimes, the error isn't about a failed scan but about a weird pointer you found. You might see THREADSTACK0-000009C0 in your address list instead of a normal base address (like Game.exe+124ABC ).
This is often the most humorous error for veteran users because the fix is so simple. In the Cheat Engine interface, there is a box labeled "Value". This is where you type the specific number you are looking for (e.g., 100 HP, 5000 Gold, etc.).