There are 2 ways to take screenshot on Samsung A21s.
With buttons and without buttons.
1 – with buttons
Press ‘volume down’ button and ‘power’ button simultaneously.
2 – without buttons
Go to settings => Accessibility => Interaction and dexterity => turn on Assistant Menu
You get assistant menu floating on your screen.
To take screenshot – tap assistant menu and you get options. In the options, tap screenshots and that’s it.
Note – Assistant Menu will not be captured unless you take screenshot with buttons.
Here is the video for visual