There are a handful of reasons that pairing a device can fail.
PhonePresenter is not Running
Check that PhonePresenter is running before pairing a device.
Not on the Same Network
Check that both the computer and device are connected to the same network.
Bad Wi-Fi Connection
PhonePresenter may not work well on slow or weak Wi-Fi connections. Move closer to or reorient the Wi-Fi router to get a better signal.
Blocked by a Firewall
Ensure that a firewall is not blocking the PhonePresenter server.
Incorrect Network Interface
On a computer with multiple network interfaces, choose the correct interface on the main window before pairing a device.