Hi and welcome to the Forums.
Seems you have a reference on the wan interface, maybe the default policy lan->wan. Try to delete this policy and all other references (e.g. ssl vpn) the you where able to put the wan port to the sd-wan interface.
It is FGT factory default that there is one switch that has port 1-n and has an internet policy via the WAN port(s).
Basically you can add any interface to sd-wan as long as it doesn't have reference(s). That is because if you add an interface to sd-wan it does no longer exist as an own interface for use in policies etc because you want to use sd-wan instead then.
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