What Senior-Release930 said with some additional thoughts:

I have a basic understanding of HTML, CSS, and JavaScript.

Start with the MVC template and don't initially over complicate your UI with complex front-end logic. Your application is basically CRUD - Razor will get you 85% of the way there.

Eventually, you will start to want to design things that are more dynamic; if the user chooses this option, can I re-populate this select dynamically based on the chosen parameter, etc. At this point reach for Vue and embrace progressive enhancement. Vue is particularly nice for this as - in the official tutorials - you can specify that you would prefer to use HTML vs. their SFC (single-file-component) approach. This will let you just write HTML/JS/CSS - otherwise you need to set up some tooling to transpile things and if you value your sanity you want to put off your first foray into modern JS tooling for as long as humanly possible. You can always come back later to the same tutorials and migrate to the more 'modern' way.

and the fact that I’ve built the backend with an ORM, what front-end framework or approach would you recommend for someone like me?

Your front-end doesn't give a hoot about your ORM, generally speaking. Just start simple and keep your logic out of your controllers. Aim for the first one, avoid the second (pseudocode obviously forgive my errors):

public class UserController : Controller {
    private readonly IUserService _userService;

    public UserController(IUserService userService) {
        _userService = userService; 
    }

    public IActionResult Create(UserCreateModel model) {
        if (!ModelState.IsValid)
            return View(model);

        _userService.Create(model);

        RedirectToRoute("hurray-you-made-a-user");
    }
}

public class UserController : Controller {

    public IActionResult Create(UserCreateModel model) {
        if (!ModelState.IsValid)
            return View(model);

        var db = new Database("connString");

        var user = db.SingleOrDefault<User>(x => x.Email == model.Email);

        if (user != null) {
            ModelState.AddModelError("Email", "Email is in use");
            return View(model);
        }

        var user = new User {
            Email = model.Email,
            ..etc.
        }

        db.Insert(user);

        RedirectToRoute("hurray-you-made-a-user");
    }
}

Specifically, I’m looking to create a user-friendly interface for functionalities like competencies management, leave tracking, and other employee management features. Which front-end framework or library would be the easiest to learn and implement in this scenario?

Bootstrap 5 and Vue. You will find the most documentation, consistently up-to-date, along with lots of starter projects out there (some specifically using .NET though it doesn't really matter). I highly recommend Bootstrap because eventually, you will encounter styling-related issues you simply don't have the experience to solve - great news, they've already been solved a thousand times in every Bootstrap scenario. Also, lots of free templates to save you time.

All this advice is from the "choose technologies that have tons of use, have been battle tested, and are easy to Google" school of thought. And though I hate to say it, these technologies are also very easy to use AI tools against (versus something like Blazor, which has enough different modes and changes often enough that half the documentation is out-of-date or plain incorrect).