You’ll need to change your navigation.

You might not have noticed many changes to Android 11 when you updated your Pixel phone last week—unless you tried to take a screenshot on your Pixel 2 or Pixel 3. Depending on which navigation method you selected, you might have lost the handy screenshot shortcut that was built into the power button.

While Google has taken away two-button navigation on the Pixel 4 and 4a, it remains on the Pixel 2 and 3, which is where the problems begin.
…
Continue here:
https://www.pcworld.com/article/3574942/how-to-get-the-screenshot-button-back-on-your-pixel-phone-in-android-11.html