How do I insert values into a Map<K, V>?

There are two issues here.

Firstly, you can't use the [] syntax like you may be able to in other languages. Square brackets only apply to arrays in Java, and so can only be used with integer indexes.

data.put is correct but that is a statement and so must exist in a method block. Only field declarations can exist at the class level. Here is an example where everything is within the local scope of a method:

public class Data {
     public static void main(String[] args) {
         Map<String, String> data = new HashMap<String, String>();
         data.put("John", "Taxi Driver");
         data.put("Mark", "Professional Killer");
     }
 }

If you want to initialize a map as a static field of a class then you can use Map.of, since Java 9:

public class Data {
    private static final Map<String, String> DATA = Map.of("John", "Taxi Driver");
}

Before Java 9, you can use a static initializer block to accomplish the same thing:

public class Data {
    private static final Map<String, String> DATA = new HashMap<>();

    static {
        DATA.put("John", "Taxi Driver");
    }
}

The two errors you have in your code are very different.

The first problem is that you're initializing and populating your Map in the body of the class without a statement. You can either have a static Map and a static {//TODO manipulate Map} statement in the body of the class, or initialize and populate the Map in a method or in the class' constructor.

The second problem is that you cannot treat a Map syntactically like an array, so the statement data["John"] = "Taxi Driver"; should be replaced by data.put("John", "Taxi Driver"). If you already have a "John" key in your HashMap, its value will be replaced with "Taxi Driver".

The syntax is

data.put("John","Taxi driver");