Hello,
Yes. SRX can not act as L2TP Client.
Basically L2TP clients are end users or host machines.
When using L2TP over IPSec, client subjects actual payload to be encapsulated by PPP, L2TP, UDP & then finally ESP header.
L2TP servers are firewalls/routers who receive ESP packets, decrypt the packets & then send them towards LAN for further processing.
But Juniper SRX devices can not act as a L2TP over IPSec end point.
Regards,
Rushi