Ensure your Checkout page is linked correctly.
- Go to WooCommerce → Settings
- Click on Checkout and ensure the Checkout Pages selected are correct
- If you have more than one Checkout page
- Click on Pages, click the Title heading to sort by name, then delete the duplicate checkout pages
- Click on Trash and Empty the Trash
- Return to the Page list and Quick Edit the Checkout page and ensure the slug is checkout
- Repeat steps 1-2 above.