Why can't I create indexes in MongoDB?

Question

MongoDB

Good evening, I want to create an index in my MongoDB database, but it is not created and throws out the following error:

Here is the index creation code:

collection.createIndex({login: data[1]}, {unique: true}, (err, result) => { // data[1] хранит в себе значение поля login
       console.log(err, result)
})

Answer 1

data[1] stores the value of the login field

i.e. what does it store?
it seems like there you can only set login: -1 or login: 1
For an ascending index on a field, specify a value of 1; for descending index, specify a value of -1.