Checkout Behavior is not implemented by Shopify as part of their Enhanced Ecommerce setup. Why? I have no idea really. It's one of several things in the default analytics setup that can be improved.
I managed to track the checkout bevahior with the attached code and it worked for a while. For several days though, any code in the "Additional GA script" box does not seem to get exectued though (I tried to add alert or console.log statements). As a result, the checkout behavior tracking also doesn't work anymore. Any clue about that?
A lot of volatility can happen when you start playing with code in the additional scripts section. Shopify developers often advise against it, but if there's something you have to do and it's the only way to do it, then you just have to take the risk. Just monitor it and know it's volatile.
Use console.log("test"); in different places of the additional scripts code. Then view the developer code to see it comes through. Check for errors there too. Lastly, remove code in the section to isolate the issue.