There are many reasons that the transaction could be failing, you really need to retrieve the response from paypal in order to know what the cause of the failure is.
on the checkout failure page, you can go to the bindings panel, and click the plus button, select eCart -> Checkout -> Local Checkout response.
The gateway list has 2 options for paypal express, select the first one, then report the steps and select the second one.
in each of these binding groups is a full Response binding. This will contain the full failure response form pay pal that contain information about why the transaction failed. Without the full response, it's really just guessing in the dark. the full response will give a solid lead on what to look for.