IntelliSence is sometimes buggy with UE4 and give red line even thru compiler compiles normally, i know it late reponce but how do you call FOnlineSessionSettings::Set? it’s not static function to call it like you presented
Don’t use UMACROs (like UPROPERTY) on non-reflected classes. UMACROs work only with stuff that is reflected with other UMACROs that error mentioned and all OnlineSubsystem classes are no reflected, not to mention UCLASS() only works with UObjects (classes with U or A prefix)