when I start to write a query in a 'Sql File' I receive the error ORA-00942 in a alert dialog for every letter I write.
For exemple: I want to write the query "select * from User", when I write the letter 's' the alert dialog with the error appears. Then I press OK and write the letter 'e' and again the alert dialog appears. This is happening for every letter I write.
That behavior is completely "off-the-wall". You do not provide any background information to help anyone who cares to respond. For example, the SQL Developer version in use, database you are connecting to, third-party extensions you may have installed, etc.
If you just installed the latest production version (3.2.20.09.87) into a empty directory, had no prior version to migrate settings from, and connect to an Oracle database, then the behavior (with Tools -> Preferences -> Code Editor -> Completion Insight on by default) when typing "select * from u" should be to pop-up a list of possible table/view names to select from.