This message will be shown when Google Play previously rejected an app update:
Depending on your app’s update status, it may not be sent for review automatically. For example, if your app update was rejected and you’ve subsequently made changes in an attempt to resolve the issue, your changes aren’t sent for review automatically. You must go to the Publishing overview page and click Send for review to submit your changes. To learn more. go to App update statuses.
To resolve this, log into Google Play and submit changes manually. Then going forward (assuming no more app rejections) you should be able to automate sending the build from Appflow to Google Play again.