Newbie with VPN

Hello,

Recently we have purchased a Fortigate 500E for our institution. At now, we use as VPN a OpenVPN machine, but we wish to migrate this service to the Fortigate.

We have configurated a interface with the range of our IP public net (193.144.XXX.YYY/24) and it’s works.

The problem is, if we want to create a VPN with a public IP we get the error, “the IP is used in other interface”.

How can we create a VPN using a IP public from our subnet public range?

THANKS!!!!!