QB Issue Resolution:

Sorry, once a file has been upgraded (i.e., from 2017 to 2019) you can’t go back.  You could open a pre-2019 backup with 2017.

Resolution for Issue 'Can I go back to 2017 from 2019' available: Yes (Solved).
Source: Intuit Community forum.

Can I go back to 2017 from 2019: 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.

Support for file
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