mongoose aggregation $group with nested object

$group can only contain _id or accumulator objects like $first, $last, $sum etc. In your case your building nested object and that syntax is not allowed - accumulator has to be on a top level. You can try two approaches, either return flat structure from $group and then reshape using $project:

{    $group: {        _id: '$level',        level: { $first: '$level' },        passageId: { $first: '$passageId' },        userId: { $first: '$userId' },        type: { $first: '$type' },        category: { $first: '$category' },        score: { $first: '$score' },        completedStage: { $first: '$completedStage' },        userPassageStatsId_id: { $first: '$_id' },        readingTime: { $first: '$readingTime' },        qtdVocab: { $first: '$qtdVocab' },        qtdTestDone: { $first: '$qtdTestDone' },        totalQuiz: { $first: '$totalQuiz' },        progress: { $first: '$progress' }    }},{    $project: {        _id: 1,        level: 1,        ...,        userPassageStatsId: {            _id: "$userPassageStatsId_id",            stats: {                readingTime: "$readingTime",                ...            }        }    }}

or use $$ROOT to capture first object for every group and reshape it using $project:

{    $group: {        _id: '$level',        d: { $first: "$$ROOT" }    }},{    $project: {        _id: 1,        level: "$d.level",        ...,        userPassageStatsId: {            _id: "$d._id",            stats: {                readingTime: "$d.readingTime",                ...            }        }    }}