I am trying to buy an app from The App Store...

I came back off vacation and found that my iMovie app was corrupted, and so I was advised by the error to uninstall and get it again from the AppStore.

However, every time I try, after putting in my Apple ID and password, I get this pop-up as per the attached image below...

I saw that a way to resolve this is to add payment method and shipping address. Shipping address is fine but it won't let me add a payment method...

I don't have the ability to add a payment method as I didn't add one when I created my Apple ID...now the add payment method field is unclickable and doesn't allow me to add one...

How do I fix this please?