If any interface is already used in the FortiGate configuration, you cannot add it to the SD-WAN interface. In this case, you must delete any security policies that use either WAN1 or WAN2, such as the default Internet access policy. Traffic cannot reach WAN1 or WAN2 through the FortiGate after you delete the existing policies.
Also check for any other references to WAN1 or WAN2 and make the necessary modifications.
If you have many policies that reference WAN1 or WAN2, you can redirect those policies to unused ports rather than delete them, so that you don't have to recreate those policies again. You can redirect those policies back to the SD-WAN interface when it is created.
- Go to Policy & Objects > IPv4 Policy and delete any policies that use WAN1 or WAN2.