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