Welcome to the Community, @sharonhouca.
It's my pleasure to help you get past this error so you can successfully activate your QuickBooks Desktop 2018.
If you're getting the validation code error or unavailable service, you can perform some steps to resolve this. Let me route you to this article for the detailed steps: Fix errors when activating QuickBooks, or adding license and product numbers.
You don't need to follow all the solutions provided. Once you've performed the first solution, you can go ahead and activate your QuickBooks again to check if it works. If you get the same error, you can proceed to the next solution.
However, if you're getting an error 3371, please refer to this article to resolve the error: Error 3371: Could not initialize license properties.
For other common errors you might encounter, you can check out this link for the detailed steps: Resolve installation errors by deleting damaged file.
Let me know how it goes. I'll be around if you need further help with the activation. Have a good one.
