b’coz there is no connection to the DB that the Universe is using. When you get that error message click on Details and you will get a detailed error message. Find out the error message there and make sure the Universe connection to the DB is set properly. Open the Universe in Designer and go to File - > Parameters and click on the Test or choose Edit and make sure the given details are correct.
When you get the SQL Sentence error message, there is often a ‘Details’ button, which if you press it may give you an Oracle error code? Have you got a button and is there a code - if so what is it?