Shop subdomain: the value can be found from your dashboard’s URL:
In that image, a4niaj-jz is the value.
Then in your Shopify dashboard, go to Settings → Apps and sales channels → Develop apps (would ask to be enabled the first time) → Create an app → Enter app name → Create app → API credentials → Install app → Copy the Admin API Access token and paste it in “Access Token” field in n8n → Scroll down in Shopify and copy the API secret key and paste it in “API Secret Key” field in n8n → Go back to Shopify’s app settings → Configuration → Edit the Admin API access scopes and select the API scopes you need.