This is the solution. I needed to convert the enum to a Byte:
TEnumAsByte<EObjectTypeQuery> ObjectToTrace = EObjectTypeQuery::ObjectTypeQuery7;
TArray<TEnumAsByte<EObjectTypeQuery> > ObjectsToTraceAsByte;
ObjectsToTraceAsByte.Add(ObjectToTrace);
if (GetWorld()->LineTraceSingleByObjectType(HitResult, startPos, endPos, FCollisionObjectQueryParams(ObjectsToTraceAsByte))
{
}