I am unable to open an excel workbook that I created in a site on SharePoint online. Each time I click on it, I get the error "Couldn't Open the Workbook The workbook cannot be opened." This is the case even when I create a workbook on excel desktop then upload it to SharePoint. The uploaded copy fails to open and displays the same message.
@Brutefaux do you have the same credentials (password & user login) in your Excel Desktop App as in your M365 Login online?
I think your online created Excel Workbook does not find the same user acount data in your Desktop App. Pls. check your Excel Desktop App on your Notebook or PC, if the right Account is registered in there, you should be able to open that SharePoint file. You could only have an online Excel licence, without the permissions to work in the Excel Desktop App in that special file.