The button is coded to check if there is any animation data present. If there isn’t any, then it will be greyed out.
It also checks the frames, so if your animation imported and somehow the start/end is set to 0/1 then the button will be grayed out.
Let me know if that helps.
Be careful with the older rigify version, they did say that the spine was changed and that the behavior on it currently - stretching all sorts of bones including the breast bones - is a bug. I never did follow up to see if a fix was planned other then taking their advice and re-connecting the rig differently during the metarig creation process.