Google has a selection of ways that you can verify that you own the website that you want to manage.
Due to the way our platform is built the first option (HTML File Upload) is not possible. However, any of the 3 other options will work just as well.
HTML Tag - Easiest option
For the HTML tag option, simply copy the string that GMC (Google Merchant Center) gives you and paste that into the 3rd Party code under Settings > SEO/Tracking. Ensure that the dropdown is set to "Place inside HEAD tag".
Then head back to your GMC account and click the "Verify Now" button to move on.
Using a Google Account
If your Website is already verified on Google Search Console then you can simply link the two together to confirm the ownership. Note that you'll need to be an administrator on Google Search Console to be able to verify in this way.
Alternatively, you can use your Google Analytics Account, this unique ID can be placed/found under Settings / SEO/ Tracking and Tracking Services.
Simply add your Analytics Account ID (example shown above) and hit Save. Then head back to Google Merchant Centre and click "Verify Now".
Finally, you can use your Google Tag Manager account. This is the least preferred option as it requires tag manager knowledge and administrator level access.