I believe this is due to Mac El Capitan release. This started happening to me as well after the upgrade. Here’s a workaround I have. You have to check the “Save in Drafts Folder” . Then it shows it in Mac Mail with the attachment, and you can send it. […]
Resolution for 'Capitan release issue: 'The sales receipt is not being attached to the email to customer.' available: Yes.
The Capitan release issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the Capitan release issue yourself by looking for a resolution described in the article. If it is a complex Capitan release issue or you are unable to solve the issue, you may contact us by clicking here or by using other Capitan release support options.
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