It is possible that not your whole balance at Bullex is freely available to put in new orders. For example when you previously put in a Limit buy order to buy 10 ETH at 200 Euro, the 2000 Euro involved in this open order is not available to cover for other new orders.
When you put in a new order at Bullex for a specific trading pair, we will display both the Total balance as well as the Available balance you hold for both currencies involved in this pair.
It is always possible to 'free up' your balance at Bullex for new orders, by closing some of the Open orders you still have. To cancel an open order, please visit the trading page for the trading pair involevd, and scroll down. There you will find a section called 'Open orders'. Select the order you want to cancel. To the right you will see an option to click 'Cancel'.