track pad won't click on login screen

Good luck.

At the page referenced above there is a section describing adding an entry to the .GlobalPreferences.plist file. I found that I needed to add a DIFFERENT entry:

com.apple.trackpad.forceClick must be type boolean and set to value 1

Some additional information about how to do this:

1. The problem is that the user that runs the login screen has tap-to-click disabled. How it got that way I don't know.
2. I don't know which user runs the login screen but it is not the 'root' user.
3. I don't know how to log in as that user so that his trackpad preferences can be changed.
4. The master .GlobalPreferences.plist file is owned by root and can only be edited by root, even if you change its permissions to allow other users to edit it.
5. The root user is by default disabled on macOS. To enable it, a user with admin privileges must go to System Preferences, Users, unlock admin access, click Login Options, Click Network Account Server-Join, click Open Directory Utility, unlock admin access, choose Edit, Enable Root User. I think it prompts for the root user password to be created. Now, the login screen will show a new option "Other...", where you can choose user "root" and enter its password you created, to log in as the root user. But you don't need to log in as root user to fix this problem.
6. The .GlobalPreferences.plist file is not a text file but some kind of binary file. The program that was able to edit it on my machine was Xcode. Xcode is the software development GUI including C compilers etc. for Mac systems. It is a FREE program available at the App Store (you may have to sign up to be a Mac developer, which I believe is free to do). The download of Xcode from the App Store can take HOURS. But, you don't need to install or use Xcode to edit the .plist file.
7. There is another way to read and write the .plist file, using the "defaults" command from the Terminal command line. Using this you can discover what setting it is that you have under your regular username that causes the trackpad to recognize tap to click, by reading your own .GlobalPreferences.plist, since every user has his own version of that file. Make sure that in System Preferences, trackpad, point and click, that "Top to click" is checked. Then, at the terminal prompt, type the command "defaults read ~/Library/Preferences/.GlobalPreferences.plist" to get a list of all commands in the file. Look for one that says something like "com.apple.trackpad.forceClick" = 1; That is what you need to add to the same file located at the same filename except you leave off the tilde (~) character, so you get the master file in the system Library directory rather than the one in your user Library directory.
8. You can more easily search for the command in that file by adding a grep search to the above command: "defaults read ~/Library/Preferences/.GlobalPreferences.plist | grep trackpad" or replace trackpad with other words like mouse or pad.
9. You can write the command to the system .plist file by first using the "su" (superuser) command to switch from yourself to the root user. You do that by using this command at the Terminal prompt: "su root". It will prompt for the password. If it works, you are now logged in as root user and so you can modify the master .GlobalPreferences.plist file.
10. Once you successfully superuser as root, then you can modify the master file with this command: "defaults write /Library/Preferences/.GlobalPreferences.plist com.apple.trackpad.forceClick 1"
11. You should then be able to restart the computer and the trackpad will now work for single-tap clicking during login.

Make sure you enable the root user as described, or you won't be able to superuser to root.

Key items to do above are 5, 8, 9, 10.