The Superstore data sources all link to Excel spreadsheets. I've found that this error typically means that that spreadsheet no longer exists or was moved. Can you check the following path to see if the file exists there? ...Documents\My Tableau Repository\Datasources\<Version Number>\<Language>
Another option is to download a fresh copy of the global superstore data file (Superstore Data Sets - dataset by tableauhelp | data.world), save it on your computer, then edit the data source and select that file.