Aggregation failed using $in in MongoDB conditional aggregation query

I am trying to use $in in MongoDB query similar to this one

This query works for me and uses { $gte: [ "$likes_count", 1000 ] }

    db.Tweets.aggregate(
       [
          {
             $project:
               {
                 _id: 0,
                tweet: 1,
                 convert:
                   {
                     $cond: { if: 

{ $gte: [ "$likes_count", 1000 ] }

, then: "1000 likes or more", else: "less than 1000 likes" }
                   }
               }
          }
       ]
    )

However when I use {tweet:{$in:[/tesla/i,/rocket/,"coffee"]}} in almost the same query I get aggregation failed error

db.Tweets.aggregate(
   [
      {
         $project:
           {
             _id: 0,
            tweet: 1,
             convert:
               {
                 $cond: { if: 

{tweet:{$in:[/tesla/i,/rocket/,"coffee"]}}

, then: "contains words", else: "does not contain words" }
               }
           }
      }
   ]
)

I know that in a simpler query the $in works

db.Tweets.find({tweet:{$in:[/tesla/i,/rocket/,"coffee"]}} , {likes_count:1, tweet:1, _id:0})

However I am not understanding the correct way to format $in aggregation method https://docs.mongodb.com/manual/reference/operator/aggregation/in/

I have tried swapping with { $in: [ "tesla", "$tweet" ] } but this still creates aggregation failed error

db.Tweets.aggregate(
   [
      {
         $project:
           {
             _id: 0,
            tweet: 1,
             convert:
               {
                 $cond: { if: 

{
        $in: [ "tesla", "$tweet" ]
      }

, then: "contains words", else: "does not contain words" }
               }
           }
      }
   ]
)

Please help. I am still new to MongoDB and curly braces, the queries look almost the same to me apart from using $in and the $gte.