Report server couldn't connect to the data source

Created a test .pbix file with the March 2018 version of Power BI Desktop (which matches our current report server version).

The data source that I used was Web and I connected to an excel spreadsheet on our O365 sharepoint site. From the desktop, I was able to pull data fine and refresh it when I modified data in the spreadsheet.

However, once I uploaded the .pbix on to our Power BI Report server, I am unable to configure a data refresh due to credentials not working, but they are the same ones I used within the desktop.

I do not understand why this is the case. Any information would be greatly appreciated. Thank you.