Ok. So I went to a tutor and worked with them to come up with this solution to toggle my hamburger menu. Just thought I would add this answer for the benefit of other beginning React programmers.

import React,{ useState } from 'react';
import { FontAwesomeIcon } from '@fortawesome/react-fontawesome';
import { faBars } from '@fortawesome/free-solid-svg-icons';

function Nav(props) {
const [navClass, setnavClass]= useState("flex-row");
const [toggledNav, settoggledNav]= useState(true);

const addClass=() => {
  if (toggledNav){
    setnavClass("flex-row show");
    settoggledNav(false);
  }else {
    setnavClass("flex-row");
    settoggledNav(true);
  }

}

  return (
<header>

  <h2>
    <a href="/">
      Travis Helms
    </a>
  </h2>
  <nav>
    <ul className={navClass} id='navul'>
      <li className="mx-2">
        <a href="#about">
          About me
        </a>
      </li>
      <li>
        <span>Portfolio</span>
      </li>
      <li>
        <span onClick={() => props.setmenuSelect(1)}>Contact</span>
      </li>
      <li>
        <span>Resume</span>
      </li>
    </ul>
  </nav>
  <button onClick={addClass} className="hamburger" id="hamburger">
    <FontAwesomeIcon icon={faBars}></FontAwesomeIcon>
    </button>
    </header>
  );
}

export default Nav;

First of all, welcome to React! It's an amazing framework!

It would have been more helpful if you provided the error you are getting but from the code you shared I can see there are a few things you need to do differently.

Generally, you can use event listeners in React for certain things, and there is a certain way to add them properly, but for the most part they are not used very frequently. Instead, for example, to assign a click event to an element in React, all you need to do is pass it the onClick attribute with the function you want it to execute when it is clicked, for example <button onClick={doSomething}>click me</button>.

An easy way to control the state of your component is by using the useState hook and toggle the class of an element using state. In your case, it could look like this:

import React, { useState } from 'react';

const Nav = () => {
    const [isOpen, setIsOpen] = useState(false);

    return (
        <header>
            <h2>
                <a href="/">Travis Helms</a>
            </h2>
            <nav>
                <ul className={"flex-row " + (isOpen ? "show" : "")}>
                    <li className="mx-2">
                        <a href="#about">About me</a>
                    </li>
                    <li>
                        <span>Portfolio</span>
                    </li>
                    <li>
                        <span>Contact</span>
                    </li>
                    <li>
                        <span>Resume</span>
                    </li>
                </ul>
            </nav>
            <button
                className="hamburger"
                onClick={() => setIsOpen(!isOpen)}
            >
                <FontAwesomeIcon icon={faBars}></FontAwesomeIcon>
            </button>
        </header>
    );
};

It looks like you are in a very early stage of learning React and for you to be successful coding with React you need to cover a few basics like state management, props, children and a few more things. I would recommend going on YouTube and find a good introduction video to React and code along to it! There are a lot of great resources in there for people who are just starting with React, just make sure it's something fairly recent (I'd say 2020 onwards).