Seeing a transaction fail is alarming for a beginner, but it usually has a simple, fixable cause, and understanding the common ones removes the panic.
The most frequent reason is not attaching enough gas, the fee that pays for processing. If the fee is too low, or set below what a busy network currently demands, the transaction can fail to go through. Network congestion makes this worse, since fees rise when many people transact at once.
On decentralized exchanges, a trade can fail because of slippage, when the price moves beyond the limit you set between submitting and processing. Another common cause is simply not having enough of the network's coin left to cover the fee, even if you have plenty of the token you are sending.
Frustratingly, a failed transaction can still cost a small fee, because the network did work attempting it. The usual fix is to try again with an adequate fee, during a quieter period, and with enough balance to cover costs.
