Hi Alexander Aaron here from the Customer.io support team.

One way that you can achieve this is by using action conditions to prevent sending push messages to profiles without any devices synced:

• https://customer.io/docs/journeys/action-conditions/

Alternatively, you can set up a Filter and Exit conditions on a campaign so that anyone without a device (one of our built-in data-driven segments) will be filtered out of the campaign if no device is synced:

• https://customer.io/docs/journeys/campaign-triggers-and-filters/#when-are-campaign-filters-evaluated

I’d recommend the former if you have other message types (such as email) in your campaign that you’d like to send even if the customer doesn’t have a device.

Let us know if you have any other questions, we're always happy to help.