Error in Mapping when using Concat and Constant functions

Dear All,

I am trying to follow the blog(How To Use SOAP 1.1 1.2 and HTTP POST for Web Services Integration) the by Mark Dihiansan Thanks a ton for the great guide Mark. I have similar requirement so first I am just trying to mimic the same as Mark has shown in his guide to build the HTTP body, but I am getting the bellow error while testing the message mapping.

11:56:59 Start of test
Mapping object MM_Add incomplete. Unable to continue execution 

• Target field mapping '/AddIn/a' incomplete; reason: Mapping for target field '/AddIn/a' incomplete

Thanks,

Farhan