Even though there are many writers who want to write a story with someone else, there are some people who do not understand what it means to collaborate with someone else. After all, a simple request is still a request at the end of the day.
If you ask another person to collaborate with you before introducing an entire story, then taking charge as the main writer of the plot, the writer you are trying to 'collaborate' with will only feel like they are getting extra work dumped on them from others.
One of the main draws to writing is being able to creatively continue the plot in your own personal style. If all you offer is being able to write the story you thought up, then it is normal for that person to feel as if they are being taken advantage of.
Instead, before anything else, you need to make clear that both parties have full control over the plot rather than one person being in the drivers seat. I know this would lead to having to talk to each other a lot should your creativity clash, but that's just how a collaboration between two writers works.
You can't expect two people to be folly compatible in a collaboration all the time. After all, some people like leaning into drama while others like action that hit a person off their seats. If both parties aren't able to talk conflicts in creativity out, then there is no point collaborating with each other in the first place as you will probably never get your chapters done.
Now, after establishing that fact, you can come up with a first chapter from scratch after agreeing on the type of story you want. From here, ground rules must be established and followed accordingly.
What role do you want to take in the collaboration and what responsibilities are you willing to share?
My advice is either let one person become an Editor who alters their final draft or take turns writing chapters each. The former can be done even if the latter option is chosen as you can work together.
Both of you can take turns editing a former chapter and building up the story from the events that happen in the chapter. As you both write, you can even discuss things you want the other to add in their chapters while speaking up if they add something you don't want to the story.
This might seem simple and straightforward, but not many establish such simple ground rules despite how it normally leads to one side backing out. To discuss things with each other is a simple process now that we have Discord and Amino these days.
Anyway, that's all I got to say. Be sure to ask me if there's anything else you want me to analyse~ ✌️