If your single-use virtual card does not have sufficient amount of funds to make a payment you need to:
Go to "Requests" tab
Delete the previous request
Click to create a New Request, this time requesting a higher amount making sure there will be enough funds to cover the payment.
If your recurring virtual card runs out of funds for an upcoming purchase:
Go to "Subscriptions" tab
Click on subscription in question
On the right side panel scroll till you see "Action" button
Click on "edit monthly budget".
Make sure that monthly budget > purchase to come.
If your physical card runs out of funds, request a top-up from the Avatar tab > Physical Card (Admins or Account Owners), Card (other roles) or My Card (on mobile).