Make sure you understand what pairing before continuing troubleshooting by reading this article.

This article contains list of recommended steps to pair a terminal.
Recommended steps differ based on different state the terminal is in and a different version it has installed on it.

This guide assumes you have sever version at least 20230801.

Jump here to right section that applies to your case:

My terminal has version 20230801 and higher

Terminal state

Actions to take

Notes

Terminal is unpaired or never has been paired

and

terminal operates in Deployment Scenario A

This case applies for example to new terminals that has been purchased.

Create terminal on server.
Generate VPN configuration for terminal if you use Deployment Scenario A
Set on terminal server’s host/ip via advanced administration. Terminal should show error screen with pairing code is displayed.
On server in CAS new pairing request will appear on terminal list page.
Approve the pairing request.

If terminal displays “VPN connection error” it means that you probably didn’t generate VPN configuration.

My terminal has newer version than 20190130 but has older version then 20230801

My terminal is version 20190130 and older

Troubleshooting pairing

My terminal has version 20230801 and higher

My terminal has newer version than 20190130 but has older version then 20230801

My terminal is version 20190130 and older