If you have a payment in your payout dashboard as "Available Now":

The business has paid you for the Gig. You can Instant Pay this money if you have a debit card as your payout method. Only funds that are listed as "Available Now" can be Instantly Paid.

If you have a payment in your Payout Dashboard "On the Way":

Stripe has automatically transferred the funds to your personal account, which they do every business evening between 9pm and midnight if there are funds in your Stripe account to send.

Once funds are "On the Way" they cannot be Instant Paid.