Hello @gujingc ,
Thank you for getting contacting us & sorry for the inconvenience caused.
Please find the answers to your questions below for the errors that you have faced.
1. Sorry, this product cannot be purchased message: This message will be displayed only if the product is restricted to purchase from the WooCommerce or the price, type or any other required information is not added while creating the product. For example, while creating a product you might have missed to add the price for the product.
So, can you please check that you have added all the necessary & required information in the product setting. Also, can you please check that is that product can be purchased from the Default WooCommerce’s Checkout page or not. Does it displays the same message?
2. No product is selected message: This message will be displayed only if your flow is not in the test mode as well as you have selected any product in the backend setting of the checkout page under Select Product Tab.
So avoid this error either you put the flow in the test mode or select the product in the backend setting that you want to sell.
On the same note, please remove the flow from the test mode before going live with the flow.
I hope this helps.
Please let me know in case you need more help.