(Big spoiler below:)
Try examining the flower Peyton gave her!
(if this doesn't work plz let me know, haha)
Hmm. Yes it should be a key item.
I'll look into it on my end and try to replicate it + push out a fix. Unfortunately you can't get the ending without it.
(I have an idea of what caused it. If at any point you returned to the main menu and then went and got that scene with Peyton, it's possible that the flower wasn't added to your inventory. )
Okay bad news first: I can't seem to replicate your bug on my end!;;;
Good news: I build a special build just for you!
Go ahead and download the build called "Peyton's Flower".
When the game opens, click on the button in the upper right to start the game.
This will load a save that has all the variables ready for the ending. If your inventory is empty, don't worry. Go ahead downstairs and click on the iron door. I've added in an extra bit of code in this dialogue that will drop the flower into your inventory.
Discover the new room and activate the ending (this will make the title music stop playing, haha). This time you'll have the needed item for the ending!
If you're playing on Mac or Linux, let me know and I can make this build in those operating systems for you!
Thank you again for your patience!
(And if anyone else is reading this and has also gotten this bug, feel free to also download this build!)