Follow the steps below to enable SMS consent at checkout on Shopify.
Step 1: Log in to your Shopify account and go to Settings at the bottom left.
Step 2: Go to Checkout.
Step 3: Check the "Show an option to subscribe at checkout" box.
Under Checkout, scroll down to the Marketing Consent section and check the "Show an option to subscribe at checkout" box.
4) Click on Save.
And you're done! Customers completing your checkout process will now be asked if they want to subscribe to your SMS Marketing campaigns.