Check out this thread:

https://community.intuit.com/questions/1101578-can-i-see-if-a-customer-has-opened-the-emailed-invoic…

Resolution for 'Check out issue: 'Where can I confirm that an email has been sent to a client?' available: Yes.

The Check out 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 Check out issue yourself by looking for a resolution described in the article. If it is a complex Check out issue or you are unable to solve the issue, you may contact us by clicking here or by using other Check out support options.

For more information about resolving issues related to Check out in Quickbooks/QBO or to get support for the issue 'Where can I confirm that an email has been sent to a client?', click the link below: