Chaining of Fabric Attach Proxy switches is not supported.
You can connect FA Clients to the FA Proxy (or FA Standalone Proxy) but not other ERS FA Proxy switches.
If you really needed to hang the ERS3600 off the ERS5900 stack, and you are running Fabric Connect, you could let the ERS5900 run FabricConnect, then it would become a FA Server, and you would be allowed to have ERS3600 FA Proxy connecting into it.