Prevent customers from adding a pre-order product with normal products in the same cart using the Pre Order for WooCommerce plugin. To apply this setting, follow the steps below:
Go to WooCommerce > Settings > Advanced Pre Order and Backorder > General Settings.
Enable Prevent Mixing Products setting to avoid mixing pre order & in-stock products in the cart.
Click ‘Save Changes’ to apply the setting.
Click ‘Save Changes’ to apply the setting.