When I export, it says this layer can’t be exported, what’s wrong with it?
Maybe there is a little error sign in the lower right corner after you click on OK.
This might give more information when clicking on it.
Also, the log file might be helpful. You could attach it here.
How to find the log files? - SNAP FAQ
Maybe a simple typographic error: in your screen snapshot the Save Image box appears to have a space character after the period: