This error seems to come up quite often, and many people have difficulty figuring out what it means, so let's break it down.
"entry point scripts"
In NetSuite terminology, an "Entry Point Script" is the module you write that will be assigned as the source file on a Script record.
If your module requires an @NScriptType tag, then it is most certainly an Entry Point Script. Another common Entry Point is a Client Script that will be attached to a Form by a Suitelet or a User Event beforeLoad. Although this type of Client Script doesn't necessarily have a Script record, it is still very much an Entry Point.
"script type function"
All Script Types provide an interface - a predefined set of functions which a developer can implement and NetSuite will implicitly understand which system event will invoke said function. For example, the User Event script type provides an interface with the beforeLoad, beforeSubmit, and afterSubmit event handler functions.
Your User Event could export any other number of functions, but NetSuite would only understand what to do with exactly these three.
"entry point scripts must implement one script type function"
Replace the error message with our analysis of its components, and you have something like: "Your Script module must implement one predefined event handler."
Rephrase that in the context of a programming error, meaning there's a problem or something you haven't done correctly: "Your Script has not implemented a predefined event handler."
This is exactly the cause of the problem. All entry point Scripts must implement at least one of the predefined event handlers for that Script type, which is precisely what the original error message states. You can implement one event handler, some of them, or all of them, but you cannot implement none of them.
To fix your code, make sure you have at least one of the Script type's predefined events connected to a function within the return statement of your module:
If you don't actually need the entry point to do anything - as is often the case with a Client Script that only serves as a button click handler - then just attach an empty function to one of the events.
Analytical thinking like this is something you must be capable of in order to be a competent, confident SuiteScript developer - my coaching program is intended to help you get there.