According to Intuit, Error Code 21 indicates that a code registry entry could not be found. This error can occur when installing or using the integrated QuickBooks PDF Converter, the driver licensed by Intuit from from Amyuni Technologies.
The most common fix for this error is to re-install the PDF Converter, which involves deleting the QuickBooks PDF Converter and attempting to email any form or report. QuickBooks will automatically reinstall the PDF Converter after you’ve deleted it.
This error is tied to a long series of similar errors associated with printing to a PDF using the integrated driver.
For more information on deleting and re-installing the PDF Converter, consult this Intuit knowledge base article.
For more information on resolving Error Code 21, consult this Intuit knowledge base article.
If the steps outlined in that article don’t resolve the error, Intuit also offers these basic printer troubleshooting tips.