Here's how to let a client off a cancellation fee. There's no separate waive button: you take the fee off the invoice, or give the money back as credit or a refund.
Where the fee is
A cancellation fee is a line on an invoice, like "Cancellation fee: Walk, Thu 12 Nov (Milo)". It's on the invoice the appointment was already on, or on a new invoice of its own.
Before your client pays
Tap Invoices and open the invoice under Unpaid.
Under Services, tap the cancellation fee line.
Tap the delete button at the top left and choose Remove.
The invoice saves itself with the new total. If the fee was the only line, delete the invoice instead: tap ... at the top right and choose Delete.
On Android, tap the fee line, scroll down, tap Remove and confirm. In the web app, open the invoice, click the remove button on the fee's row under Services, and save.
After your client has paid
Give them credit for the fee:
Tap Clients and open the client.
Under Credit notes, tap Add credit note.
Enter the fee as the Amount, and a Reason, like "Cancellation fee waived".
Save it. Your client is told they have credit, and you can use it on their next invoice.
To give the money back instead, refund the payment. See How do I refund a payment?
If they paid when they booked
The fee was kept back from their refund when the appointment was cancelled. To waive it, open the invoice, tap the payment under Payments, choose Custom Refund and enter the fee. See How do I refund a payment?
Don't charge it in the first place
When you cancel an appointment yourself, choose Don't charge when Walkies asks about the fee. See How do I cancel an appointment?
Good to know
Only admins and sub-admins can change invoices, add credit or refund.
Taking the fee off doesn't bring the appointment back. Book it again if you need to.
To stop charging fees, turn off Apply cancellation fee in Cancellations. It only changes cancellations from then on. See How do I set a cancellation fee?
For more on credit, see How do I give a client credit?