As understood, the problem statement is that you are unable to add an L2L tunnel for a new Site on your firewall with some existing VPNs already working.
I would like to have an opportunity to help you fix this issue. I have got more than 10 Years of experience in Networking domain and have been managing multiple VPNs across the globe on multiple devices mostly Cisco ASA firewalls.
In my current role, I am managing more than 25 tunnels setup in each of my 20+ firewalls across different offices, though, for most of the tunnels only one end is managed by me and other end by one of my customer's network engineer.