Hello Carol,
What version are you using?
If you are using Simply Accounting 2008 (or lower) go to Setup, Reports & Forms and click on Invoices. Make sure in the Email Form Settings the Invoice description is NOT set to User-Defined Invoice. If it is, change it to Invoice or one of the other basic templates and test the email again. This time it should work.
In the 2008 version and lower you are not able to email a custom simply form, it has to be a basic template. You would need crystal reports to customize an email form template OR upgrade to the 2009 version of Simply Accounting to email a custom simply form.
If the error persists chat back with the specific error message (I'm making a bold assumption here!)