The exact error, explained
What is the #NULL! error in Excel — and how do I fix it?
Seeing this message? It's more specific than it looks — and usually a quick fix. Drop your file in below and it'll be checked for you, no reading required.
What this message means
This error points at a specific problem in the file, not at your account — which means it's something you can fix and re-upload.
Why it happens
Most often this comes from how the spreadsheet was saved or edited — a stray character, a wrong delimiter, a column that got renamed or dropped, or a value in the wrong format.
How to fix it
You can hunt for it by hand, but the fastest path is to let the tool at the top of this page read the file and point straight at the offending rows or cells — then hand it to your agent to correct in one pass.