Error: The selected file is read-only and can therefore not be opened.

The error message means that the project file of MAXQDA is stored in a folder where your computer use has no right to write. 

This is typically the case if you open the MAXQDA24 file directly from your email program, like Outlook. 

When opening a MAXQDA project file from an email attachment, please don't open the file directly, but save the file first on your local hard drive. 

Only when you open the project file from your local hard drive, you will be able to open your project file.