Error - Electrum wallet RPC method not supported
There are two problems with your command.

First, any method you call should not be prefixed by wallet. or suffixed by ().

Also, there is no RPC method create_new_address. If you have a look at lib/commands.py you will see that the internal python method create_new_address() is called by the RPC method getunusedaddress, and only when the parameter force=true is given.

With the above in mind, try this:

curl --data-binary '{"id":"curltext","method":"getunusedaddress","params":{"force":true}}' http://127.0.0.1:7777


The equivalent shell command would be

electrum getunusedaddress --force


I tried this on electrum 2.8.3. Sadly, it always returns the same address. This is because the address remains unused.