That is the error phrased for the user. There is a log somewhere given a more technical description. In sprintr.mendix.com look for the log of the application, and you best start looking in sprintr.mendix.com, click on Apps, click on your App, click on menu item Environment and then click on button View live log.
The ‘App from spreadsheet’ feature in the Create New App flow was temporarily unavailable.
But this has been restored and should work again.