I think it is quite obvious from the error message that you may have a line of data with a value in debit column but negative in amount_currency or a value in credit column but positive in amount_currency.

Or maybe you have a line of data with values in both debit and credit?

Well... depending on your accounting principle... basicly there is two ways of posting policy, contra and storno
odoo uses contra postings, wich does not alow nehgative amounts (if negative ammount appears it is transfered to kontra side debit/credit)
also.. some countries (china, poland, croatia, bosnia, serbia.... don't know all of them) use storno postings... wich allow negative ammounts on debit or credit side. If that is your case, and you are trying to import negative ammounts , in order to use it you will probabbly need at least account_storno module. (google it or search on github or launchpad repos) 
hope it helps