Let's try to remove and re-add the connector as that can often fix this.
With the template open in edit mode, click Resource->Manage added data sources. Take any of the sources you were trying to use with this template and click remove. Then click "Add a data source" to go to the connector selection page. Find under "Community Connectors" the one you were using. Highlight it and 3 dots should appear next to it. Click on that and click Remove.
Now the connector should be totally removed. Search again for the connector to re-add it. (If the "explore connectors" option shows nothing, try https://developers.google.com/datastudio/connector/gallery/). It will have you go through the authentication and configuration process again for the connector. Continue through the process to save the data source.
Once the connection and data source is set, you can go back to the template and update the Data Source field for the scorecards and graphs to the updated one you just created and hopefully the errors for the trial are gone.
If you run into other errors or issues during any of the above steps, please let me know.