QB Issue Resolution:

That appears to be happening on some systems.  Are you using Excel 2008?  So far that has been a common element.

Resolution for Issue 'QBooks 2015 for Mac reports no longer exporting to Excel (2008) ever since updating (per Intuit email) to V16.1.7 R8 before updating Mac OS to Sienna' available: Yes (Solved).
Source: Intuit Community forum.

QBooks 2015 for Mac reports no longer exporting to Excel (2008) ever since updating (per Intuit email) to V16.1.7 R8 before updating Mac OS to Sienna: 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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