Begginer need help please

Hello,
I’m new on git hub and I just want to use Nvidia haireworks.
But I don’t know whhy I can’t to do a pullrequest.

All step I was doing :

On the epic game/unrealengine

  • clic fork (I’ve selected, upload -> unreal 4.16.3, Because it’s the version supposed to work with hairework)
  • branche master

After on the Haire work

  • clic fork
  • branche HaireWorks

After on the hhairworks page.
-pull request
-New pull request
on the lesft I selected unreal/master on the right haireworks/hairworks

A message appear : “** Can’t automatically merge. **Don’t worry, you can still create the pull request.”

I have serched somme tutorial (written and video) but I don’t know… I’m probably just too stupid to understand.
If someone have any idear about what am I suppose too do for it Work, and can take the time to explain it to me, it make me… very very happy.

ps: sorry for my bad english.