If I open the app, it has the same issue i.e. a warning message appears suggesting I need to 'sign in to save this file'. Additionally, in the top right corner there is also the 'Sign In' and 'Buy Microsoft 365' buttons. So it just looks like is still doesn't recognise the fact I've installed a brand new version of 365.

Either that, or it's an issue with 'One Drive'. Each time I try save to OneDrive and I enter my hotmail email address it just spins back to the Excel file.