I’m trying to test out payments before I go live on my site and I get an error message when I click the add to cart button. I looked into what the problem could be by using W3C Validator. It showed me a couple of errors but I don’t understand enough to fix them. Can someone help?
If your store is still on a trial plan without any paid plan chosen the checkout will not be available.
The checkout itself can’t be altered unless you are on the plus plan, which means that the issue may be related to the cart page/drawer if the checkout is active on this store.
You also need to make sure you have a payment option selected as well.
Sorry you are facing this issue, it would be my pleasure to help you.
Welcome to the Shopify community! 
Thanks for your good question.
Please share your site URL,
I will check out the issue and provide you a solution here.\
thanks for more details
Here is my site. Its not live at the moment so you would need a password to view it. https://treasuredcrafts.co/
Thanks for any and all help @KetanKumar and @Shay
yes , please password
@KetanKumar vithay is the password
your have set up your payment or contact Shopify help center