Create Pull Request Button is disabled

In my case, the pull request Title field didn't auto-populate like it usually does. Once I typed in a title, the button became active.

From Github Developer Support:

Also, I know this might seem strange, but could you try selecting the base repo/branch from the drop down list (even if this already seems selected), and give this another go. It could that, for whatever reason, the pull request creation flow isn't picking this up implicitly.

I have also logged out and logged in again.

After that, everything was fine!