The "Aero" setting is a setting in Windows that allows applications to use advanced graphical effects such as transparency. The Untapped.gg Companion requires this setting to be turned on so that it can integrate seamlessly into the game.
Close any game you might be running and exit the Untapped.gg Companion by right-clicking on the icon the taskbar and selecting "Quit"
Open the "Control Panel", and visit the "System Page"
On the left hand rail, click "Advanced System Settings"
Open the "Advanced" tab in the window that just opened
Click the first "Settings..." button (in the "Performance section")
Select the "Adjust for best appearance" option, in the "Visual Effects" tab
Click the "Apply" and then the "OK" button
Start the Untapped.gg Companion again by searching for and clicking it's shortcut
You can now start MTGA and should see the companion appear in the game.
If the overlay should still not be showing, follow these steps here.