Notion API Key
The Notion API key is found in the Developers Notion Page. For more detailed instructions, see the Notion API Integration - Getting Started Page.
Notion Database URL
Enter in the full URL of the Notion Database and not the ID only. If the URL of parent page of the Database is used, you will be prompted to choose which database you want to use. This can occur if you create a Database - Inline or Database - Full Page in which the database becomes a child of that database. For more detailed instructions, see the Notion Database - Getting Started Page.