The first thing to try when Rectangle isn't working

[rxhanson]

Sometimes the accessibility permissions in macOS get out of sync. This can happen upon macOS updates. When this happens, Rectangle's commands for snapping don't work, and the menu items are greyed out.

To get it back in sync

1. Close Rectangle if it's running
2. In System Settings > Privacy & Security > Accessibility, first disable Rectangle, then remove it with the minus button. (it's important to do both of those steps in that order)
3. Launch Rectangle and enable settings for it as prompted.

If you're handy with the Terminal you can execute these terminal commands instead.

killall Rectangle
tccutil reset All com.knollsoft.Rectangle
open -a Rectangle

If it's still out of sync, you might have to logout/login.