Google Play Store Internal Test cannot start roll out
Have you filled out all required data under the things with gray check boxes? All 4 checkmark icons should be green and if you hover over a gray one it will pop up what is remaining e.g upload icons, feature graphic, etc.
For those who may be coming to this question later, this button may be disabled if you recently signed up for new Google Play developer account.
You might be seeing a message like this:
This is due to a change by Google to keep bad developers out of the store. (an article)
This can be the case even though you're ready to roll out the changes.
Seems like filling out all the fields is a requirement per the step 3 of the following page to rollout any release:
https://support.google.com/googleplay/android-developer/answer/7159011?hl=en