Google Play Store Internal Test cannot start roll out

Solution 1:

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.

Solution 2:

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.