See this, https://community.intuit.com/questions/1415388

Resolution for '2016 upgrade issue: 'Why does the 2016 upgrade default to the QB Server app? I don’t use it' available: Yes.

The 2016 upgrade 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 2016 upgrade issue yourself by looking for a resolution described in the article. If it is a complex 2016 upgrade issue or you are unable to solve the issue, you may contact us by clicking here or by using other 2016 upgrade support options.

For more information about resolving issues related to 2016 upgrade in Quickbooks/QBO or to get support for the issue 'Why does the 2016 upgrade default to the QB Server app? I don’t use it', click the link below: