Thanks for reaching out to the Community for assistance, @jahve. 

I'm here to help you remove the error message that you are receiving on the invoice. 

Since the message is advising us to make the customer active again, let's click on the blue hyperlink that shows on the screenshot you provided above.

If this doesn't help you activate the customer again and you're still receiving the error message on the invoice, then I recommend clearing your browsing data. This will allow QuickBooks to load as a fresh new page that removes any error messages that may be standing in the way. This is a link that shows you step by step on how to clear your file data. 

Remember, I'm only a comment away if you have any more questions.