Porque está tratando de usar el $lookup
características (sintaxis) de MongoDB v3.6 en MongoDB v3.4
El MongoDB v3.4 $lookup
sintaxis:
{
$lookup:
{
from: <collection to join>,
localField: <field from the input documents>,
foreignField: <field from the documents of the "from" collection>,
as: <output array field>
}
}
El MongoDB v3.6 $lookup
sintaxis:
{
$lookup:
{
from: <collection to join>,
let: { <var_1>: <expression>, …, <var_n>: <expression> },
pipeline: [ <pipeline to execute on the collection to join> ],
as: <output array field>
}
}
https://docs.mongodb.com/manual/reference/operator/ agregación/búsqueda/
