If you see this error after saving grades, it could be due to a mistyped custom code or stray characters in the score cell.
For example, you have a custom code "t/" but you entered "t\". If stray characters are the problem, it can be harder to see. Click inside the cell and keep tapping the right arrow key. You may only see the score you enter in the cell but the right arrow key will show that you may have entered something like this: "38 3".
In all cases, just fix the score and save your grades. The error will go away at that point.