I have a Power Automate flow where I am using the Get manger(V2) action. Get manger(V2) is an action that queries for the Manager of a given M365 user. When you are using the free M365 Developer tenant, you can end up with a global admin with no manager name (see the third screenshot below). Unless you fixed this ahead of time, creating a flow as a global admin can lead to this kind of error.
The solution is to make sure that the user using your flow has a Manager assigned inside the M365 tenant/Azure AD.
=> Below is the screenshot you see inside the Power Apps flow designer:
=> Below is a screenshot of the email you get as that flow user:
=> Below is where you have to add a manager for your user. Note: You need to be a global admin to do this.