You should check if the site is a group connected site. When it is a group connected site you could use the Delete Group method from the Graph API in a Send an HTTP request action of the Office 365 Groups connector.
DELETE https://graph.microsoft.com/v1.0/groups/{id}
If it is a modern site (Communication sites and Non-group associated Team Sites) you should be able to use the _api/SPSiteManager endpoint in a Send an HTTP request to SharePoint action.
url: /_api/SPSiteManager/delete
method: POST
accept: application/json;odata.metadata=none
odata-version: 4.0
body:
{
"siteId":"d11e59ca-1465-424c-be90-c847ba849af5"
}
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