When your Swap order amount is very close to your total balance for that particular crypto asset, your order will sometimes fail. This is because of market movements that may cause the value of your order to increase beyond your available balance.
Cryptocurrency markets can be volatile which means that fluctuations in asset values are inevitable. These fluctuations can increase or decrease the value of a crypto asset.
In this scenario, the best course is to resubmit your order with the desired amount in crypto rather than fiat.