If your single-use virtual card does not have sufficient amount of funds for the payment to go through, 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 the subscription in question,
On the right side panel scroll till you see "Action" button,
Click on "Edit monthly budget",
Make sure that monthly budget is superior to the 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).