Hi, this is most probably caused by ports and protocol being blocked. Technically, these are ports UDP/500, UDP/4500 and ESP protocol.
We would recommend you to ask the network administrator or your Interned provider, they will know what to do.
You can also try to open the ports in your modem settings yourself, but this is often really complicated and requires advanced skills.
Last but not least, some countries (like China or most of the Arabic states) block VPN in general, so if the user is from some of these, there is probably nothing he/she can do to connect to the VPN.