The full message reads: "There's a problem with this formula. Not trying to type a formula? When the first character is an equal (=) or minus (-) sign, Excel thinks it's a formula." Excel shows it when you press Enter on something it cannot read as a formula.
The seven causes, and the fix for each
1. Text that starts with =, - or +
A note like -- check with finance looks like a formula. Type '-- check with finance instead.
2. The wrong argument separator
Your regional settings decide whether arguments are separated by commas or semicolons. A formula written for one setting fails on the other. Change the separators to match your Excel.
3. Curly quotes
Copy-pasted formulas often contain “curly” quotes. Retype them as straight quotes.
4. An unmatched quote
Text in a formula needs a quote on both sides: =IF(A1="Yes",1,0).
5. An unmatched bracket
Count the opening and closing brackets. Excel colours each pair as you type, so the odd one out stands out.
6. A sheet name with spaces
Use ='Sales Data'!A1. Without the apostrophes Excel stops reading at the space.
7. A missing operator
=A1 B1 has nothing between the references. Add the operator you meant: =A1+B1 or =A1&B1.
Still stuck?
Paste it into the free Formula Explainer or Excel Error Solver, or send it to the free Formula Clinic. All guides are in the Excel error library.