I tried preordering thru pc app but the pre-order button is greyed out did i missed the preorder window?

I'm not entirely sure, having only used the app since the second, when I got 2 months of the Xbox Game Pass Ultimate for 2 dollars. But the button is grey, logging out so I could look at what the store page looks like without having to try preordering, the button just is grey (the border changes, highlighted it turns a shade of grey, not highlighted it's white.) It seemed to work fine for me, although I didn't go past the point where it asked for my password when I clicked on the preorder option.

I'm not entirely sure what it was the app downloaded, but I was able to install some 10mb placeholder with my Gamepass, so I think it will automatically acquire it once the release version is available? If so, thanks for giving me cause to poke around on the Outer Worlds page of the Xbox app, because I likely wouldn't have done that again for a couple of weeks or more.

There are no preorder bonuses, so there really shouldn't be a "preorder window" of any kind, I can safely say you didn't miss it. You might consider just picking up the Gamepass for awhile