The Lot size is the minimal amount of currency that you can buy or sell.
For each market, there's an established lot size. For example, the BTC lot size is equal to 0.01 BTC.
The most common issue when you attempt to buy/sell a certain amount of assets is that the desired quantity is less than the lot size.
There are two options to resolve this:
You can withdraw these assets or just store them in your account.
You can buy some more of this currency specifying the exact price.