QB Issue Resolution:

Before sending your statements, you’ll see the Send Forms window. In this window, you can review the statements you want to send and change your email template or message. Once you’re ready, you can click the Send Now button.

 

If there is any missing information, like an email address, a dialog box will pop up. You’ll need to supply the correct information before you can proceed. The entire email process will be canceled if you click on the Cancel button.

 

I’ve attached a few screenshots of how it looks like.

Resolution for Issue 'Email STATEMENTS to multiple customers at once' available: Yes (Solved).
Source: Intuit Community forum.

Email STATEMENTS to multiple customers at once: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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