ERROR in compilation : template argument X is invalid | working with boost::beast

I am trying to make websockets using C++ to connect to IBM web server. I am using the BOOST library. And I am refering to the Vinni Falco's CPPCON 2016 websocket example code for testing the initial connection.

Upon building the code I have got many errors of similar kind which is "template argument 3 is invalid". Attaching a screenshot of the same.SCREENSHOT

I have looked into the header file(s) to which it/they are pointing to.Please refer this for the highlighted error But I am unable to figure out the solution to the problem. I have read and referred to Tutorials Point and a reference PDF which explains the basics of how to use templates.

Any help to resolve this error will be appreciated. Thank you

Rohan