"Unknown "craft" function" Error in Plugin Service Call
There are multiple issues happening here...


The Twig code should look like this:

{{ craft.businessLogic.exampleVariable() }}

As noted in that example, Twig can't communicate directly with a Service level method. You'll need to ping the Variable class, and use your Variable method to call the Service method.


Optional...


The Business Logic plugin template currently only works with Craft 2. If you're attempting this on a Craft 3 site, several things would need to be changed.