Greetings lynnefromesher,
Thanks for using the Apple Support Communities!
I understand that you are not able to deauthorize all computers because the button doesn’t appear when you view the authorized computers. If you see your old computer as the only computer authorized, this would be the reason you do not see the deauthorize all button.
The Deauthorize All button appears only if you have more than 2 authorized computers. If you don't see the Deauthorize All button, that means that you only have one authorized computer.
Deauthorize your computer using iTunes - Apple Support
You will be able to authorize this computer without deauthorizing the old computer first. The following article will walk you through the process of authorizing your computer.
Authorize your computer in iTunes - Apple Support
Have a great day!