How do I add a custom textarea into the checkout screen? I want to add 'Notes' textarea for adding addtional message by customer while placing the order.

I have added additional textarea in 'Order types' page (/admin/commerce/config/order-types/default/edit/fields). It is displaying in order page but not in checkout page.

Example: Customer is placing an order for Toys and they want to add message saying 'Please deliver in 2 days..'

Edit: I have seen same question in Drupal 7. But there is no 'Commerce Fieldgroup Panes' module in Drupal 8.
You will need to write a custom checkout pane plugin, I'm afraid.

Docs here: https://docs.drupalcommerce.org/commerce2/developer-guide/checkout/create-custom-checkout-pane