When you add an Excel Online card to a flow for the first time, you'll be prompted to create a new connection.
You must enable Microsoft Graph to use Excel Online. See Excel Online API documentation.
You can create multiple connections and manage them from your Connections page.
To create a new connection from an Event or Action card:
Click New Connection.
Add an Connection Nickname. This should be unique, so that if your teammates authorize multiple accounts, then you will be able to tell them apart.
Click Create to launch a pop-up window to sign into your account and save this configuration.