This plugin does work with the twentytwentyone. In fact we have a whole test site dedicated to the twentytwentyone theme.
It is worth noting that out of the box the twentytwentyone theme does not contain any widget areas besides the footer. As themes define their own widget areas, there not much this plugin can do about adding sidebar support to a theme.
If you are unable to see the widgets in the footer, please see the FAQ section which explains the most common reasons why a widget does not show up.
