If you are looking to sell (an) asset(s) from your portfolio, but only see a possibility to buy, there might already be a pending sell order for this security. You can check this in the Order book menu, where you can still modify or cancel your order as long as it has not yet been executed.  

Even if, for example, assets have been used as collateral for options, you will not be able to sell them. If this is the case, please contact the Order Desk to check if the collateral can be adjusted.