In the first step of the authentication process, you will need to enter the store name you want to login. This is the first part of the store's URL, i.e. 'mystorename' of mystorename.myshopify.com
In case you accidentally enter the whole store URL, or any input with a dot(s), you will end up with 403 Forbidden -error.
If you ended up with the 403 Forbidden -error, just close the browser tab and go back to Supermetrics for Google Sheets and click the Shopify data source on the side panel.
In the first part of the authentication process, please enter only the first part of the store name without dots.
Example of authorising with a valid store name
Example of authorising with a faulty store name
You may be prompted a security alert, depending on your browser setup
In case you continue here by claiming the path is safe, you will get 403 error