It is impossible to say what is going wrong with your installation without a single detail describing your current process.
I guess the main things that come immediately to mind would be:
- Install Windows first because both systems will need to share the EFI partition. I’ve never had a problem doing this with Linux but I have heard Windows will sometimes wreck the contents of an existing EFI partition during installation.
- If you can, avoid legacy boot. Best to disable it completely in the BIOS menu if possible.
- Secure boot must be disabled.
There are tons of threads in the forum about this topic. If you get stumped, read through a few of those and see what other people have done to get it to work.
This might be a good place to start: