Except for the custom codes that you create, and the letter E (exempt), if you save anything other than a number in a Gradebook cell, this counts as a zero.
This article provides a resolution on how to fix such alerts with entry errors or undefined custom codes.
Most likely, the problem is a typo such as a space between digits, a comma, or a mistyped custom code.
Correct the erroneous entries and save the scores again.
Scores may exist in assignments not currently displayed on the main Gradebook spreadsheet or for dropped students which are hidden.
In these cases, use the arrows next to the "Go To..." button to view more assignment or the "Show Dropped" button to view the dropped students (picture below).