Is monitor traffic interface lt-0/0/0.0 supported ?
Connection is UAR, ping works to, but this command isn't providing any output, it is some sort of issue or monitor traffic interface on lt and vtep interfaces isn't possible ?
monitor traffic only shows traffic reaching the routing engine. lt- and vtep interfaces forwards traffic on the forwarding plane so not reaching the routing engine.
To monitor data on lt- or vtep I think you need to look at port-mirroring (expecting you are using an MX): https://www.juniper.net/documentation/en_US/junos/topics/usage-guidelines/services-configuring-port-mirroring.html