Authenticate API calls in R

I'm not familiar with R, but it looks like your header doesn't have a key, for the value.  The Rest API will look for the base64 value you're creating in the "Authorization" header

Where do I get the key? I am not sure if I understood correctly.
I have passed Authorization header in base64 format.

The rest API uses Basic Authentication and should be formatted accordingly; 

Authorization: <type> <credentials>

Where type is Basic and credentials are a base64 of admin:admin:location:password

Just to elaborate a bit on what James said, here are some notes I made about authenticating the api:

Each REST API call must be authenticated with a username and password. The username consists of three fields concatenated together: "UserName:UserGroup:MachineLocation". 

1. The UserName field is set in the Users table of the Tessitura Security Module.
2. The UserGroup field is also set in the Tessitura Security Module.
3. The MachineLocation field is set in the TX_MACHINE_LOCATION table in Tessitura.  You can add your machine name or pass any existing machine name from that table. From an API perspective this is simply an audit column, but it is checked against that table at authentication.
4. The password is created for the UserName in the Users table of the Tessitura Security Module. The checkbox for "User must change password at next login" must not be checked!
   1. NOTE: the user type defaults to "user".  Set the type to "api" to prevent the password from expiring.

Bravo Steve.  Much more eloquently stated;