"Error message, cannot access system"

Okay, it sounds like you have a multitude of problems here, let’s try and answer them. 

With your screenshot earlier, it looks like you were copying files from one location to the Dropbox folder. The error you got was from Windows, saying it couldn’t copy the file. That error is unrelated to Dropbox, since as it states, the original files could not be accessed by the system. 

Now, if that has been resolved, your second question is about the email used on your Dropbox account. In each of these places, we want to find the email address used to connect to Dropbox. Please also look carefully for typos in your email address in any of these places. 

1. On your computer, click on the small Dropbox icon in your menu bar or system tray. Click on the gear in the Notifications panel. Click on Preferences, then click on Account.
2. On iOS, open your Dropbox app and tap on "Account". On Android, open your Dropbox app, tap on the Menu button, and then tap on "Settings".
3. On the Dropbox website, click on your avatar or the face in the top right corner. 

Lastly, is the app showing that it is up-to-date? Does it say syncing? Can you navigate in your Dropbox folder through the blue syncing icon to see where the problem files are?  

Thanks in advance!