There are three possible reasons you might not be able to short sell in your account.
1) Your account is a cash account. Short selling is only allowed in margin accounts.
2) You have less than $2,000 total account equity in your margin account.
3) The security itself isn’t available for short sale. This is usually because the security is a low-priced security that isn’t shortable or because we can’t borrow the security from the market and lend it to you due to current market supply and demand.