Help/Assessment Wanted For A Vaccine Registration Form Validation

Hello, I am new to coding and would greatly appreciate some help cleaning and fixing up my code.

I made this vaccine registration form after trying to learn from MDN Form_Validation using JavaScript DOM, I don’t want to use the HTML attribute “required”, because that wouldn’t practice me interacting with the DOM. Complete CSS shall be added later.

I am currently encountering some issues that made me almost sleepless for 5 days already and it’s clearly taking a toll on me.

My current issues. Or things I want to do for now.

  1. I have a radio button that asks, “Did you already get your second vaccination?”. I want to create a function or form validation that actually only activates if the user selects the “yes” radio button.
  2. I have a popup modal that shows the results for users to confirm the details after the user clicks the “submit” button. My issue with this is, the “back” button that’s suppose to close the modal doesn’t work, while the function that allows you to close the modal if the user clicks the overlay on the other hand works.

I would be really grateful if everyone here can lend a helping hand.
I would also like to apologize in advance if my spaghetti code is one heck of a mess.

Due to restrictions, I am unable to attach the files. Here’s my CodePen Link instead.