Creating applications using Power Apps and Power Automate is straightforward. When creating an app in Power Apps, sign in to https://make.powerapps.com, and for Power Automate, https://make.powerautomate.com. You can select the template needed for your app or flow or create it from scratch. You need to create a connection if you want to store your data in a data source.
A connection is a connector to communicate between the data source and your app or flow. There are more than a thousand connectors in Power Platform that you could leverage. SQL to SharePoint, Outlook, Adobe, AvePoint Cloud Governance, and more. After selecting your data source and adding a connection, your app and cloud flow is up and running. Then, you realize you need to move it to a Solution.
By selecting Add existing, you can easily move your existing app, cloud flows, and other components into a solution.

After adding your flows to the solution, you will see a warning from the Flow Checker. The details are about using the Connection Reference, which you need to use in the solution.

The flow will still be running, but after a few days or so, the flow will no longer be running. You open the flow to check the error and see this notification.
“Some of the connections are not authorized yet. If you just created a workflow from a template, please add the authorized connections to your workflow before saving”

To fix this issue:
In the Flow Checker, select the Remove connections so connection references can be added.
Your flow will then refresh and have a warning icon in all your data sources. This means that you need to add the connection reference.

When you open up the Connections, you need to add a new Connection Reference. Select the + New connection reference.

Sign in

A new connection reference will then be added. Do this in the remaining Connections with warning icons. After you’re done, you can save it, and it will run successfully.

Note: You will encounter this error if you add an existing flow into the solution. If you create your flow inside the solution, it will automatically create a Connection Reference.
Connection Reference:
From docs.microsoft.com, Connection Reference is a solution component that contains the connection information. Canvas app and flow are bound to connection reference. When creating an app or flow in the solution, you use connection reference, not connection (I know it’s confusing).
Summary:
Creating an app or flow in the solution will automatically create the connection reference. You need to check the connections and change them to connection references if you want to add them inside the solution.