If you cannot quit an application or do not know how, hold down the Command and Option keys, and then press Esc on your keyboard. To quit an application, select the application on the toolbar (top), and then select Quit. You must quit all programs before you continue. To resolve this issue, try the following methods, in order, until the issue is resolved. Programs that are running in the background are interfering with Excel for Mac.
This error may occur if one of the following conditions is true: Microsoft Excel has encountered a problem and needs to close. When you try to start Microsoft Excel for Mac, you receive the following error message:
For more information about this change, read this blog post. Office 365 ProPlus is being renamed to Microsoft 365 Apps for enterprise.