I entered the same state abbreviation on more than one line of box 15, and that is what showed on my W2. Could you please help to see what I can do? Thank you very much.

It depends.  Usually this type of reporting affects local taxes.  In TurboTax, you do not re-enter the same state information in the state taxes line for the reason you state above (a diagnostic error).  You will, however, enter each additional line for local tax amounts to different localities.  You will be able to have several local taxes from the same Associated State, but the box 15 entry for state taxes will only be entered once for that state.  Please note the screenshot below for more detail.