Solution
SystemLink Enterprise (SLE) does not allow for the use of basic authentication (i.e. providing a username and password combination encoded in base64).
When a user logs into the web UI using a browser, only Single Sign-On (SSO) is allowed.
When interacting with SLE programmatically, you must provide an API Key as the authentication method.
Refer to Creating an API Key for instructions. In your code, it is necessary to add the x-ni-api-key header to the HTTP request, and provide the API Key as the value.