normally this message appears when the price of the items in your cart has changed or the amount of available stock has changed. you need to clear your cart and add in the item you need again back into the cart.
this happens especially for promo item with limited quantity but you didnt manage to pay on time but the item still stuck in your cart but actually it is already out of stock.

have you try to place other item? if all items are giving you the same message, you can either remove the app and reinstall or contact shopee.