Hi @eps1 welcome to the /e/ forum.
You will have found the answer when you have read this thread !
I recommend the solution starting at Post #8.
Regarding Documentation Suggestions
Enter suggestions to improve documentation in this category. Do not raise bugs or issues here…
As you already opened Problem with installing to oneplus 9 pro please tell me there if you are able to extract dtbo.img and vendor_boot.img from your /e/OS image. Good luck.