If you are unable to add app blocks in your Sections using the Theme Customizer, it could be due to your theme not supporting this feature or your Shopify version being lower than 2.0. Don't worry, you can still offer preorders, but partial payments won't be available without the app blocks.
If you need to offer partial payments but can't add the necessary app blocks, disable the "Shopify Purchase Options" in your settings.
Navigate to "Settings."
Under Default Settings, disable the Shopify Purchase Options.
Click Save at the bottom of the screen.
After saving, click "Sync Selling Plan with Settings."
Then go to Products in your Shopify admin.
Select the product with the variant error.
Ensure the Purchase options are disabled for this product.
By following these steps, you can continue to offer preorders and adjust your settings to manage partial payments effectively. If you need further assistance, please contact Shopify Support.