Can’t login to iMac, because wireless keyboard not found thus I can’t enter password. Is there any way to turn on onscreen keyboard at startup...so I can at least get logged in?
The information below, which applies macOS Big Sur 11.0, was taken from this Apple website.
Turn on accessibility options in the login window on Mac
You can turn on certain accessibility options in the login window to make it easier to log in to your Mac.
On your Mac, choose Apple menu > System Preferences, then click Users & Groups.
If the lock at the bottom left is locked 🔒, click it, then enter an administrator name and password. If your Mac supports it, you can also unlock it using Touch ID or your Apple Watch.
Click Login Options, then click Accessibility Options.
Select the accessibility options—such as VoiceOver or Zoom—you want to turn on in the login window.
The options you select are enabled in the login window for all users of your Mac.
If you select the Accessibility Keyboard or Switch Control option, your current keyboard or switches settings are applied in the login window for all users (until another user turns the options off or on in their Login Options preferences).
If you change your keyboard or switches settings in Accessibility preferences and want your changes to apply to the login window, display the login accessibility options in Users & Groups, then click Apply.
Anyone who logs in to your Mac can temporarily turn the options on or off in the login window by pressing Option-Command-F5 (or if your Mac has Touch ID, by quickly pressing Touch ID three times) to display the Accessibility Shortcuts panel.
If you use an option in the login window and would like to use it whenever you work on your Mac, you can set the option in Accessibility preferences. On your Mac, choose Apple menu > System Preferences, then click Accessibility.
Change Shortcut preferences for accessibility on Mac
Set Accessibility preferences on Mac
Control your Mac and apps using Voice Control
Use Switch Control on Mac
Use the Accessibility Keyboard on Mac