Hello DXB9, welcome to the Apple Support Community !
It is not possible to cancel an Apple Card (Apple credit card) by email or WhatsApp for security reasons. However, since you are abroad and cannot call, you can do so entirely in writing via the Messages app (iMessage) on your Apple device, open the Wallet app, tap your Apple Card, press the three dots button (or the profile icon), then choose Message. This will open a secure discussion with a Goldman Sachs advisor to whom you can request the cancellation of your card. You can also go through the Messages app by looking for Apple Card support (Business Chat). See you soon :)