DaviCoins are what businesses give back to their customers. They are not a payment method at the checkout — there is no DaviCoins button there. What you can do is see them, and ask to cash them out.
See a business's coins
Open a business and tap the coin icon in its top bar. That lists the business's staff and the coins attached to each of them.
You cannot request a payout from here. That is done in My referrals.
Open My referrals for your balance
Tap your profile photo, then My referrals. Each campaign you are part of shows its own card with how many coins it has earned you.
Tap the cash-out button on a campaign
The button is on the campaign card. With no coins on that campaign the button is disabled and explains underneath that there is not enough balance yet.
Fill in the payout form
Enter the amount you want and the account details to send it to.
Read what the button is telling you
The request button stays disabled until the form is complete, and the line above it says exactly what is missing — an amount, an amount within your balance, a payment method, its details.
The message changes as you fill the form in and disappears once the button becomes usable. A dead button with no explanation is a bug, not a rule.
Send the request
With everything filled in, submit. The request goes off for processing.