Intuit does not officially support converting Enterprise to QBO (esp. if you are using advanced features), but see this article:

https://help.quickbooks.intuit.com/en_US/kb/import-into-quickbooks-online-from-quickbooks-enterprise…

Resolution for 'converting Enterprise issue: 'When converting from QB enterprise 2015 to QBO, why does the screen “Export to Quickbooks online” not provide the option to choose my existing new QBO company?' available: Yes.

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

For more information about resolving issues related to converting Enterprise in Quickbooks/QBO or to get support for the issue 'When converting from QB enterprise 2015 to QBO, why does the screen “Export to Quickbooks online” not provide the option to choose my existing new QBO company?', click the link below: