The problem is that the Metal Card does not support Google Pay at the moment.
Due to this you cannot add the card to GPay, only by using the button "Add to GPay" in the bunq app. By clicking the button a virtual card number is created in background and this one will be added to your GPay wallet.
When adding a card to Wear OS you have to select a card in your Google Wallet but your Metal Card is not available for GPay and you are not able to get access to the virtual card, that is created when adding the card to the GPay wallet.
So we need support for the Metal Card in GPay to solve this issue.