The error message "This interface is used by SSLVPN IP Pool" is displayed in SonicWall when attempting to change the IP address on one of the SonicWall interfaces. The error message is shown below in the screenshot.
The physical or sub interfaces are being used in the SSLVPN | Client Settings page in the GUI for leasing DHCP IP address to the SSLVPN clients.
NOTE: Error message would still appear even if SSLVPN is disabled on the zones.
Step 1: Login to the SonicWall web management.
Step 2: Navigate to Network | Interfaces, check for a free interface and configure it with an IP address on a different subnet.
Step 3: Go to SSLVPN | Client Settings, under SSLVPN Client Address Range for Interface - Select the newly configured interface and click Accept. 
How to Test:

