This means that a JavaScript function couldn't be found. Maybe customized templates are being used and not all required JavaScript files are loaded correctly.
To resolve this, make sure that the following files are included in the template and are loaded correctly:
- all JavaScript files from the respective module (see the directory "js" of the module)
- the default JavaScripts from Magento (e.g. for the checkout).
Technical background
The JavaScript functions are usually called like this:
object.function()
If a function is not found in the given object, it results in the above TypeError. For more detailled information, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Not_a_function