Elias here with Shopify Support. Thank you for reaching out.
You can enable order notes on your cart page for customers to leave a note when they check out. The order notes is located on the Cart page, which can be accessed while customizing your theme through the Storefront editor. From there, simply navigate from your Home page to the Cart page and configure the section - Cart pages.
Here's a screenshot from my test store with the Venture theme:
Let me know if this is what you're looking for?