Three things could be wrong here:
- You have selected that 'The journal line needs approval for posting' in your journal set up. (Read more here.)
- You have put your journal lines 'On hold'. (Read more here.)
- The sum of your debits and credits is not zero. Check whether you have an offset account on each line.
Note that you can add the 'On hold' and 'Approved' columns to your journal lines by using out short cut key 'ALT-F' or using the Layout/Edit buttons.