Welcome to the Make community!
Your structure appears to be correct.
However!
That is just a JSON string representation of an array, but it is not a variable of type array.
You need to convert the JSON string into an array using the Parse JSON module.
Here is an example of how to map it:
You might need to “wrap” your array with a variable:
{
"array": INSERT_ARRAY_JSON
}
If you do not want to mess with JSON strings, you can use an Array Aggregator module instead.