A common mistake in payment approval at the Orders module is the lack of inventory for a particular SKU included in the cart. It can occur when an order is approved and the item reservation had been previously released.
In such case, when you try to approve the payment in Orders, you will see the following error:

To confirm that this is the problem, in the order details page, click View Interactions
to see the order details.
If there is an error such as "The requested reservation 00-abc could not be guaranteed for the 00-xyz order", it is because one or many of the SKUs included in the cart doesn't have available inventory, which prevents the purchase from being approved. In this case, correcting the inventory should allow the order to move on.
For more information, see Checking error details in the orders.