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 a 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.