I am unable to run tests on my code using NUnit framework, how do I properly reference it?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using NUnit.Framework;

namespace Iteration1
{
[TestFixture]
public class IdentifiableObjectUnitTests //Rename this appropriately.
{
    [Test]
    public void TestAreYou()
    {
        IdentifiableObject id = new IdentifiableObject(new string[] { "fred", "bob" });
        Assert.IsTrue(id.AreYou("fred"));
        Assert.IsTrue(id.AreYou("bob"));
    }

    [Test]
    public void TestNotAreYou()
    {
        IdentifiableObject id = new IdentifiableObject(new string[] { "fred", "bob" });
        Assert.IsFalse(id.AreYou("wilma"));
        Assert.IsFalse(id.AreYou("boby"));
    }

    [Test]
    public void TestCaseSensitive()
    {
        IdentifiableObject id = new IdentifiableObject(new string[] { "fred", "bob" });
        Assert.IsTrue(id.AreYou("FRED"));
        Assert.IsTrue(id.AreYou("bOB"));
    }

    [Test]
    public void TestFirstID()
    {
        IdentifiableObject id = new IdentifiableObject(new string[] { "fred", "bob" });
        Assert.IsTrue(id.FirstId == "fred");
    }

    [Test]
    public void TestAddID()
    {
        IdentifiableObject id = new IdentifiableObject(new string[] { "fred", "bob" });
        id.AddIdentifier("wilma");
        Assert.IsTrue(id.AreYou("fred"));
        Assert.IsTrue(id.AreYou("bob"));
        Assert.IsTrue(id.AreYou("wilma"));
    }
}}

These are the errors I'm getting:

"The name 'Assert' does not exist in the current context"

"The type or namespace name 'NUnit' could not be found (are you missing a using directive or an assembly reference?)"

"The type or namespace name 'Test' could not be found (are you missing a using directive or an assembly reference?)"

"The type or namespace name 'TestAttribute' could not be found (are you missing a using directive or an assembly reference?)"

"The type or namespace name 'TestFixture' could not be found (are you missing a using directive or an assembly reference?)"

"The type or namespace name 'TestFixtureAttribute' could not be found (are you missing a using directive or an assembly reference?)"

Note: these errors occurred multiple times, so I'll attach a screenshot so as to not cause clutter.

Error Messages