@Patsefirst() has a column parameter that defaults to _value. After pivoting your data, the _value column doesn’t exist. All you need to do is provide a column name to first that does exist:
The brackets are used to represent different Flux composite data types:
// An array or list (collection of values of the same type)
["string1", "string2", "string3"]
// A record (collection of key value pairs where the key is always a string
// and the value can be any data type
{foo: "bar", baz: 12}
// A dictionary (collection of key value pairs where the key can be any data type
// but all keys must be the same data type. Values can be any data type, but all
// values must be the same data type.
[1: "foo", 2: "bar", 3: "baz"]
You can learn more about Flux data types in the docs: