'Start rollout to beta' disabled in Play Store Developer Console

Make Sure the following are checked

Saranga is right from my experience. Before I could send out a beta version I had to ensure I had the images under store listing included. Just look at the side bar (see image) and see whats greyed out and thats where the problem was for me.

To see what still needs to be done, you can hover your mouse over the grayed out checkmark. There will be a popup that tells you what you still need to finish.

Update : The answer below is outdated. These days you should see a list of options on the left, that have grayed out ticks in-front of them. Hover over the tick and it should display the list of tasks you need done to complete that category. Visit each of these items and complete them (make sure the icon becomes green - check the screenshot below) to enable the button.

Not sure whether this is the actual answer or what you are expecting, but I am stuck at the same place (I just created an account, application and a release. Saw the disabled button and started googling). However there is a link on the top right side of the screen (barely visible, i should add), with the text "Why can't I publish?". And when I click on it I get list of error messages(I think). So as of now i am stuck on that list. Probably, fixing them might enable the button.