Keynote

If you are on macOS Big Sur then before you try all sorts of other things simply go to System Preferences > Software Update and update to the latest version, currently macOS Big Sur 11.4.

The latest version of Keynote (11.1) and other iWork apps won't run on older versions of Big Sur.

SG

Try resetting Keynote:

complete all the following steps in the order listed:

1. ensure you have a current backup of all your user files, saved to a separate drive
2. close all applications
3. uninstall Keynote; for this procedure to work, an application removal tool must be used, App Delete has a free trial and works for this procedure. It can be downloaded from the developer’s website here:  App Delete
4. empty the Trash (Finder > Empty Trash)
5. restart the Mac (Apple Menu > Restart);  Immediately after the Apple startup sound, press the shift key on the keyboard, release shift when the Apple icon appears. Start up will take longer than usual as the Mac is performing a software repair of the start up drive
6. After the Mac is fully powered up, restart the Mac normally:   Apple Menu > Restart
7. reinstall Keynote from the Mac App Store
8. Restart the Mac again and test Keynote