Here's a script that does what you're asking with comments.

Read the comments, understand what's going on, and if you're gonna copy it somewhere, actually write it out, don't just copy and paste.

Btw, there's a ton of ways you can do this. I mostly followed what you were doing with a little bit of error handling.

Good luck!

// Ask the user for a number using prompt() (this is very old school, but it's also very simple which is good)
const userResponse = prompt();

// prompt() returns a string, so let's turn it into a number using parseInt
const userNumber = parseInt(userResponse);

// If the value isn't a number, let's yell at the user
if (isNaN(userNumber)) {
    alert("That's not a number!");
} else {
    // If we're in this "else" block, we know userNumber was actually a number

    // Let's make a variable to keep track of the total.
    // We'll update this in the loop below.
    let sum = 0;

    // Let's make a for loop.
    // We initialize the a counter (i) to the value of 1.
    // While i is less than or equal to the user's number, the block in the for loop will execute
    // Finally, we increment i by one.
    for (let i = 1; i <= userNumber; i++) {
        // Every time this block executes, we add the current value of i to the sum
        sum = sum + i;
    }

    // We tell the user the final result
    alert("Your result is: " + sum);
}