Big Sur won’t recognize iMac Pro Bluetooth keyboard

After updating to Big Sur on my 2017 iMac Pro, I’m stuck at the point when I need to enter my administrative password. Although the iMac recognizes my Bluetooth mouse, it recognizes no input from my Bluetooth keyboard. I can’t enter any characters at all.

I checked the keyboard just before updating; it was charged to 67%.

If I can find an old USB keyboard & plug it in, might that work? Help!