We need more info to help, Marija.
1) What is at the END of the error message? Can you scroll all the way to the bottom and show us that?
2) What database are you connecting to?
It is possible that the SQL is too complex, or that the Tableau Server does not have enough access to the database.
It's very difficult to diagnose the issue based on your screenshot and the information in this thread. Your best bet is to open a case with Tableau Technical Support.
Be sure to attach a set of logs to your case in order to help the engineers solve your problem: Quick Start: Generate a Snapshot of Server Logs