QB Issue Resolution:

V12 and V18 are only 1 version apart. (sigh marketing) So There should be no problems in upgrading. just make a backup in 12, and restore that backup in 18.

Resolution for Issue 'How to upgrade from POS Version 2012 to Version 2018' available: Yes (Solved).
Source: Intuit Community forum.

How to upgrade from POS Version 2012 to Version 2018: 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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