You don't want pages like checkout or cart indexed given those are pages that relate specifically to a single customers session. Google isn't going to add to cart or go through checkout so you're not missing out on anything.
For my own curiosity, why do you want them indexed?
Thanks for your reply Jason.
I'm facing the following problem. My client has a good Shopify store https://coffeehit.co.uk/, where we run Google Ads and we are missing a potential for growth as something is wrong with our robots.txt file.
When I set up Dynamic Search Ad campaign on Google Search network and enter clients' website URL for targeting, it gives me the following error - http://prntscr.com/u121rm
After that I've contacted Google Support, and they said that in terms of campaign set up everything is just fine, and they suggested to check robots.txt
I also found a topic on it in Google Ads support - http://prntscr.com/u122kb
I run ads for other Shopify stores and we don't have issues like that with search campaigns. So hope someone will help us!
Look forward to your replies.