Whenever I've used an Excel file as a PowerApps DataSource, it was a local file (not Excel O365). You might try downloading a copy of the file locally, then connecting it to PowerApps.
If you want a cloud-hosted Excel file:
- Save the file to OneDrive then View/DataSource and add the OneDrive Connector to your app
- Select the Excel file then the Table to use
Just a heads up, Excel-based DataSources, especially those hosted in OneDrive can be quite finicky. There are file locks and sync issues that can make things challenging.
I'd recommend only using this type of setup for testing OR use a local Excel file for static data that rarely changes.
Move to Sharepoint as an intermediate DataSource, and finally to a formal database as a best practice.