Have you created a security definition for the security you have? Have you associated this definition with the Investment Account you want to purchase the security in?
Setting up securities and Investments is a multi-step process. Investment accounts can contain one or more securities and each security can be in one or more investment accounts. When you have an investment account without any securities associated with it the only thing you can do with the accounts is transfer money into and out of the account or set an initial balance i.e you can only do cash type transactions.
The articles in the following section of the knowledge base may assist: