Claim your website by going to Google Merchant Center and click on Business information > Website > Claim Your Website
From the options near the bottom, click on HTML Tag and copy the meta tag
Log in to BigCommerce and go to Advanced Settings > Data Solutions, here click on site verification tags.
Add your Meta Tag and Save.
Go back to Google Merchant Center > Business information > Website > Claim Your Website and click on Verify and Claim.
Claim your website by clicking on the claim button.