Had the same issue and tried all of these fixes but it was something so simple I was embarrassed. All you have to do is created a selection in your photo THEN you can select generative fill, it's grayed out because there's no selection. At least that was my problem.
Why is the Generative Fill button greyed out if I'm over 18?
If you find that Generative Fill is greyed out for you and you're over the age of 18, it is possible that you may not have a valid Date of Birth associated with your Adobe ID account. To update your Date of Birth, you can do either of the following:
Sign out and sign back in to both the Creative Cloud Desktop app and the Photoshop (beta) app
Log into Behance with your Adobe ID and enter your Date of Birth.