https://support.google.com/android/thread/76992133/can-no-longer-save-screenshots-after-android-11-update?hl=en

Edit: I finally managed to fix this issue! I'll leave this post up for anyone with the same problem. When I updated something went screwy with the permissions. I solved this by connecting my phone to a computer, opening the Photos folder, and deleting the original Screenshots folder. This forced the device to make a new Screenshots folder that it can access, and I can take screenshots again.