Few notes,

1. Javascript's slice function is lowecased.
2. At the end, when you alert the output, your variable must be the same case as you declared

Run the snippet below, to see the desired output.

var Name = prompt("Enter your name: ");
var firstLetter = Name.slice(0,1);
var finalFirstLetter = firstLetter.toUpperCase();
var remainingLetter = Name.slice(1,Name.length);
var FinalRemainingLetter = remainingLetter.toLowerCase();
var captalisedName = finalFirstLetter + FinalRemainingLetter;
alert("Hello, " + captalisedName);

This is the error message your code runs:

{
  "message": "Uncaught TypeError: Name.Slice is not a function",
  "filename": "https://stacksnippets.net/js",
  "lineno": 13,
  "colno": 24
}

Make sure you check the console when you run your code to see if any errors are being thrown, it can save a lot of headaches.

The function is not capitalized, so it would look something like this:

var name = prompt("Enter your name: ");
var firstLetter = name.slice(0,1).toUpperCase();
var remainingLetter = name.slice(1, name.length).toLowerCase();
var capitalizedName = firstLetter + remainingLetter;
alert("Hello, " + capitalizedName);

I would also try to follow some sort of naming convention to reduce capitalization errors. In the example above I'm using the javascript standards, which are camelCase for variables and functions, and UpperCamelCase is reserved for data types and classes.

Wiki for naming conventions

Here is the same code with const , Have fun learning JS!

const name = prompt("Enter your name: ");
const firstLetter = name.substring(0, 1).toUpperCase();
const remainingString = name.substring(1).toLowerCase()
alert("Hello, " + firstLetter + remainingString);

Maybe you could replace var with const. Sometimes this also solves the error.. Like this..

const name = prompt("Enter your name: ");
const firstLetter = name.substring(0, 1).toUpperCase();
const remainingString = name.substring(1).toLowerCase()
alert("Hello, " + firstLetter + remainingString);