The error message "You can't use `\eqno' in math mode" commonly arises in ShareLaTeX documents due to earlier mistakes in the code. A frequent cause is a missing closing brace '}' at the end of an equation. To resolve this issue, it's essential to review the document for any syntax errors that may precede the error message, as these can lead to confusion in math mode. Correcting these earlier mistakes should eliminate the error.