I created a custom textarea, but whenever I try and preview the site, the textarea is greyed out and I can’t type anything in it.
EDIT: So I noticed that when I add the textarea outside of the product form, it works normally. But when it’s inside it doesn’t work.
Sorry you are facing this issue, it would be my pleasure to help you.
Welcome to the Shopify community! 
Thanks for your good question.
Please share your site URL,
I will check out the issue and provide you a solution here.
I apologize for late reply. Here’s the link to the website. I don’t know if there’s some default functionality to forms in liquid that disables custom textarea tags, not sure.