Spoilers!
That's strange, there's no way to beat the game without getting 39 out of 40 endings (Ending 37 isn't necessary to beat the game).
Getting 36 out of 40 endings should lead you to where you choose to take the cat home or not. If you choose, 'Do not take the cat home', the 'You are ready' option should now be available. Once you get Ending 36 (Endings are unaligned since the first ending you get is Ending 0), you'll be locked into the endgame endings. You can only see Endings 38 and 39 in the Endings Menu list after the credits roll.
If you're still having trouble, let me know! You can also check the guide for help!