You won't be able to sell shares in one of your state corporations (turning it into a country controlled public corporation) unless its value is at least 400 billion and its P/E ratio is 30 or below. You only have one state corporation with a value over 100 billion.
Check your Trade Strategies on your Corporations page. You may be selling product at too high a mark-up. Try lowering your sell-at prices or selling at best price.