Why Traccar Client report false "sending failure" to Gurtam Wialon?

I checked that Wialon hosting accepts tracking data from Traccar Client, as described here:
https://gurtam.com/en/gps-hardware/mobile/traccar-client

I also know that data is being sent to Wialon, because I used their id test:
http://id.wialon.net/
and correctly checked the id.

But the client is always reporting failure to send data. Why is that?

Probably because Wialon server doesn't reply back.

I have the same problem. my android device keeps reporting "Send Failed", however in Wialon server the device gets online for a few seconds then gets offline again.
Any fix for this ?

You have to ask Wialon to fix it.