Need to create and setup product for Free shipping minimum amount option.
Make sure that you have properly configured your shipping rates For free shipping.
Setup product #
Step 1 :
Go to Shopify admin > Products > Add product
Step 2 : Configure Product as per screenshot.

Setup in general settings. #
Step 1 :
Go to theme customize > theme settings > general settings > Free shipping Message > Select new created product here.
Step 2 : Add your message in “Free Shipping Message” field.
Note: Use $ sign at place of price in message. $ sign will replace automatically with real price.
Step 3 : Add message to “Free Shipping claim Message”. it will show when user claim free shipping.
Step 4 : Select checkbox option where you like to display shipping message.
Save page and you have done.
