The field "$name" must be an accumulator object

There are some aggregation operators that can only be used in $group aggregation and named as $group accumulators

Just as you used $sum here you have to use for the name key as well

{ "$group": {  "_id": "$_id",  "name": { "$first": "$name" },  //$first accumulator  "count": { "$sum": 1 },  //$sum accumulator  "totalValue": { "$sum": "$value" }  //$sum accumulator}}

Accumulator is like array of Elements its Accumulates as Array.$first -> gives 1st name that goes in the group of names

Example:so if you have $_id same but different name ["Darik","John"]specifying $first will give Darik & similarly $last will give John

db.faq_feedback.aggregate({    $lookup:{       "from":"faq",      "localField":"question_id",      "foreignField":"_id",      "as":"faq"   }},{    $unwind:"$faq"},{    $project:{       "question_id":1,      "lang":"$faq.lang",      "feedback":"$faq.feedback",      "question":"$faq.question",      "yes":{          "$cond":[             {                "$eq":[                   "$feedback",                  "yes"               ]            },            1,            0         ]      },      "no":{          "$cond":[             {                "$eq":[                   "$feedback",                  "no"               ]            },            1,            0         ]      }   }},{    $group:{       "_id":"$question_id",      "yes":{          "$sum":"$yes"      },      "no":{          "$sum":"$no"      },      "question":{"$first":"$question"},      "lang":{"$first":"$lang"}   }},{    $limit:10000},{    $skip:0})

$group: {    _id:"$_id",     "name" :{ $last: '$name' },     "count" : { $sum: 1 },     "totalValue": { "$sum": "$value" }}