It sounds like there's a Placeholder in that Account, though how it would get there isn't clear. In Preferences > Investments is the "Show hidden transactions option selected?
Or, did you by mistake enter a BoughtX transaction? That too will create an "N/A" in the Cash column.
Uh oh, that sounds to me like data corruption which has also been mentioned as possibly resulting in that N/A.
I'd suggest working through the Data File Troubleshooting process here:
https://www.quicken.com/support/advanced-data-file-troubleshooting-correct-problems-quicken-windows