Why does crap books wait till an invoice is done befor it tells you that it can't be saved ?

This may be a browser issue, @ian30. 

To resolve this, let's perform some steps so you'll be able to save your invoice. Let me walk you through the steps.

First, sign in to your QuickBooks Online account using a private window. This will allow the program to run without storing cache or cookies. Please follow these keyboard shortcuts to generate a private window:

• Google Chrome: press Ctrl + Shift + N  
• Mozilla Firefox: press Ctrl + Shift + P
• Microsoft Edge: press Ctrl + Shift + P
• Safari: press Command + Shift + N

Once you're signed in, create an invoice again by going to the Sales menu.

If it works in private browsing, go back to the regular browser and clear the cache. But, if the same thing happens, you can use other supported browsers. 

Also, may I ask if you've encountered an error message? This way, I can make sure that we're on the same page so I can provide an accurate solution to your concern. 

Additionally, if in case you want to change the layout or add a logo to your invoice, you can refer to this article for the detailed steps: Customize Invoices in QuickBooks Online. 

Know that I am here anytime you need help with QuickBooks. Take care always.