In a risky first step toward issuing its own currency, the country has banned the use of Russian rubles. All transactions must now be in Ukrainian coupons, which have been in use since January. Both Ukrainian and foreign companies are dismayed by the move. The coupons, which sell for 1.45 rubles at the Ukrainian central bank, are not freely convertible to foreign currency. And executives worry about how they are going to get rubles to buy needed supplies from Russia.