If your theme does not support app blocks, you will need to add the order form fields code directly to your theme files.
Usually, this involves adding the code to the /snippets/product-form.liquid file in your theme:
From your Shopify admin, go to Online Store > Themes.
Find the theme that you want to edit, and then click Actions > Edit code.
Under the Templates header, click into the product-template.liquid file.
Locate where you would like the Custom fields to appear.
Paste the following code block:
<custom-fields></custom-fields>
Click Save.
If you are not comfortable editing your theme code, feel free to reach out to our customer support team. We are happy to help you get set up.