PowerApps and Flow are recent additions to our tenant so I'm unsure if there's an extra settings I have to enable in order for PowerApps and Flow icons to appear on any new SharePoint Online sites. Also, in Forms Settings, I only have an option to edit the list in InfoPath.
Your site will need to be using the New List Experience.
Then you will have the abiltiy to customize the forms with PowerApps.
I hope this is helpful for you.
That looks good and should be working. Let's roll back a bit...what kind of list is this that you are trying to customize?
Oh and also, it looked like from your first screen shot that you would have had some item selected in the list...the options to customize would be available if nothing is selected.
Hmmm. That all should be working. I'll need to take a closer look with a few things on my side to test, but am not at a place I can do so right now, so I'll circle back with you in a bit.
I also vaguely recall an ability to turn it off on the sites, or a permission issue, but need to research that a bit later.
Did you get anywhere on this? I just went through a few different ways of trying to actually control that feature and wasn't able to have much luck.
Share what you find!