If you go to Edit > Preferences > Investments: Is the box for Show hidden transactions checked?
If not, check it and then go to the account register screen. Under the register is Placeholder Entries visible? Click on that to view the Placeholders that might be present. If there are Placeholders shown they will need to be resolved before the account can be fully closed out in Quicken.
Simply deleting them is a good first step to try. You should be able to select Delete All making the process pretty fast. Hopefully that will resolve the issue for you.
If it does not, you might want to go to Help > Quicken Help and then type in How do I resolve Placeholder entries? There is a nicely laid out process for this.