The Suspend Button extension is officially supported up to GNOME Shell version 3.34
whereas Ubuntu 20.04 has version 3.36.x
. So rather unsurprisingly it doesn't work on Ubuntu 20.04.
On Ubuntu 20.04 you'll find the suspend option hidden under the 'Power Off / Log Out' option in the system menu:
Alternatively, you can search with the word "suspend" in Activities or applications overview; refer to this answer from Where is the suspend/hibernate button in GNOME Shell?