GiveWP version 3.17.1 introduced a feature that hides the Donate Now button when PayPal Donations is selected - the Smart Buttons are used to complete donations.

When Advanced Credit Card fields are shown (so: the credit card fields show instead of the black debit/credit smart button), the Donate Now button is needed to complete a donation.

Workaround: Please go to Donations >> Settings >> Payment Gateways >> PayPal Donations and change the Payment Field Type to Smart Buttons Only