TestFlight build on App Store Connect stuck at 'Ready to Submit'
In order to Submit an app for review you'll need to add external users from the TestFlight > Builds screen. Select the build you want to submit, and add Individual Testers from this screen. As long as at least one of them is external a series of dialog boxes will walk you through the rest of the submission.
Discovered the answer, the key was to go on iTunes Connect > My Apps > TestFlight > Any Group > Builds, and then submitted it via adding a new build in there.
In App Store Connect, this process has slightly changed. You must create a testing group in order to start external testing.
Here's how to do that:
Navigate to My Apps/TestFlight and click on Add External Testers on the left sidebar.
Give your group a name and click on Create.
Select Builds on the top and click on the little plus icon underneath.
Select your build and give the test information necessary for the review process.
