Unable to Schedule Refresh

Hi, 

I've built a report on Power BI using datasets from SharePoint and Excel. I am the author on both spreadsheets, and I am an owner on the SharePoint. Refreshing works on the desktop app, though it takes forever, but as the SharePoint updates with lots of data daily, that is to be expected. When I try to refresh manually on PowerBI on the web, which is where it will mostly be accessed by colleagues/superiors, it comes up with this error message. 

I then try to edit the data source credentials in settings, but that is greyed out, and the refresh options are greyed out too. 

I've read about gateways and stuff, but I am a complete novice at Power BI, so I wouldn't have any idea where to start if I needed one of those! I work on a company computer that's connected through a private network, so I'm not sure I'd even be able to use one of those here. But any help would be totally appreciated. 

Thanks!