Insert information in a MySQL table that contains a foreign key

The invoices table is linked to the provider table by the provider_id field, but when I insert data, I get a foreign key error, and tried in CASCADE, RESTRIC etc mode and it does not work, validate that the IDs of the tables be primary keys, the second point is that I do not know how to fill a comboBox with the data from the suppliers table so that the ones already created can be chosen in the invoice form. Who can help me?

SELECT to see the table :

//En esta instruccion sSQL se realizara el inner join de las 2 tablas proveedor y factura
        sSQL = "SELECT f.id_factura, f.consecutivo, f.id_proveedor, f.producto,"+
        " f.cantidad,f.fvalor, f.fechaini, f.fechafin, f.concepto, p.id_proveedor,"+
        " p.empresa, p.nit FROM tfacturas f INNER JOIN tproveedores p ON "+
        "f.id_proveedor = p.id_proveedor WHERE p.nit like '%"+ buscar +"%' order by id_factura";

INSERT for the table:

public boolean insertar(Tfactura dts) throws SQLException {
        sSQL = "INSERT INTO tfacturas (consecutivo, id_proveedor, producto," +
                " cantidad, fvalor, fechaini, fechafin, concepto)" +
                " values (?,?,?,?,?,?,?,?)";
        try {

            PreparedStatement pst = cn.prepareStatement(sSQL);

            pst.setString (1, dts.getConsecutivo());
            pst.setInt    (2, dts.getId_proveedor());
            pst.setString (3, dts.getProducto());
            pst.setDouble (4, dts.getCantidad());
            pst.setDouble (5, dts.getFvalor());
            pst.setDate   (6, dts.getFechaini());
            pst.setDate   (7, dts.getFechafin());
            pst.setString (8, dts.getConcepto());


            int n = pst.executeUpdate();
            if (n != 0) {
                return true;
            } else {
                return false;
            }

        } catch (Exception e) {
            JOptionPane.showConfirmDialog(null, e);
        }
        return false;
    }