If you created a Google Merchant Center account via Shopify, then you can not delete the account. You should also not try to create a new account, considering having multiple accounts is a reason for suspension.
However, you can contact Shopify to delete your Google Merchant Center account, and only do this if the account is not suspended. If it’s suspended, get it approved first before deleting it, or you will continue to get suspended with new accounts.
You should be able to access the Google Merchant Center account created by Shopify, within your Shopify account.
And I believe it uses the same email address as you used for Shopify. You can try logging in here as well: https://www.google.com/retail/solutions/merchant-center/