Update :-
Talked to a Googler working with Google Play console team regarding this issue. One misconception I had at the time was that you need to retain an APK if you want to have a staged rollout. I discussed this with him, and he clarified that it is not needed. Even in staged rollout, with no retained APK, Google play will show an install option to users who visit the app’s page. Hope this clears doubts for someone facing similar issues.
I was having the same problem and I fixed without clicking “Retain” for previous version and uploaded a new one and then “Start Rolling” button turn into clickable and done!! ^_^