Here is a guide on how to add the correct user-agents in your robots.txt file in Shopify, for the purpose of Google Merchant Center.
First login to Shopify and edit your theme code.

If you already have added a robots.txt or you see a robots.txt file in the template section, ignore the following step.
Add a robots.txt file by going to the template section and click on add a new template

At the top of the file hit enter so you have 1 clear line, and paste the following code below and click on save.
User-agent: Googlebot
Disallow: /admin/
Allow: /
User-agent: Googlebot-image
Disallow: /admin/
Allow: /
User-agent: Storebot-Google
Disallow: /admin/
Allow: /

Hi Emanuel,
Why is it necessary to add Robots.txt in Shopify for Google Merchant Center?
Thank you very much.
In case Google has trouble crawling your website.