Problem creating the interfaces for a vpn on an FW 1800 splunk

[Luis1980]

To create a VPN in a FW splunk 1800 R80.20.20 Gaia embedded, because it is not possible to create the tunnel VPN interface and create alias interface with the same IPs, via web gives me an error and says that it already exists is addressing in my topology. Someone can help me. If not explained well I can send a screenshot.

thankss.