- Why payments does fail in general ? and if failed, what shall be done, just re-initiate it ?
When you try to send a payment to a node which is not directly connected to you, your payment is routed through other nodes. Those nodes must be connected to each other. Payments usually fail if there is no route available or if the amount of coins you are trying to send exceeds the amount available in the channels which would route the payment.
Let's say you are Alice and you want to pay Dave 0.1 BTC via the Lightning Network. You didn't open a channel to Dave but you did to Bob. However, Bob has an open channel with Charlie who has a direct channel with Dave. The path looks like this: Alice -> Bob -> Charlie -> Dave. Your payment will be routed through Bob and Charlie. Depending on their fee policy, they might charge you a small fee (usually under 1 satoshi) for routing. For example, if Charlie had only 0.05 BTC on his side of the channel, he wouldn't be able to route your 0.1 BTC payment so it would fail.
- is zap still active and fully functional till nowadays as i saw the latest release was in 2019
While the development has slowed down significantly, it doesn't lack any crucial features. The app works fairly well and is user-friendly.
- is there any chance you could help me with my question about wasabi and coinjoin please
Sorry, but I haven't used Wasabi much. I might try answering it in the morning.