@DetourNoir ,
Creating that flow from SharePoint set up a basic template in your PowerAutomate list of flows.
Now you can go and edit that flow in Power Automate. The "Automate" process is not for setting up a new planner task right then, it's for creating a Flow that will allow you to create a task for a selected Item.
There will be no prompt for the assignee when you run the flow, because you need to set the assignee in the flow before it's run. This is why you need a field in your SP list that has the user's email address. You need to replace the Dynamic Content "x-ms-user-email" with the column name that has the assignee's email address.
Then, when you run the flow for a selected item, the user's email address that is on that item will be plugged into the "Assigned User ID" field of the new Planner Task.
thanks,
Kyle