I have an Apple Watch series 6 running WatchOS 7.5. I used to be able to take screenshots by pressing the Digital Crown and side button simultaneously, but this no longer works.
In Settings > General > Screenshots, "Enable Screenshots" is enabled. However, when I press the Digital Crown and side button simultaneously, my watch doesn't take a screenshot. Instead it appears to register the press on the Digital Crown first (so for example, if I'm on the home screen it opens the App view), then the side button a moment later (i.e. shows the app switcher).