interface{} to []byte conversion in golang

In your sample code data should be JSON-encoded (in simple word String) so you are using Unmarshal in a wrong way:

// Unmarshal parses the JSON-encoded data and stores the result
// in the value pointed to by v. If v is nil or not a pointer,
// Unmarshal returns an InvalidUnmarshalError.
func Unmarshal(data []byte, v interface{}) error

Try it on The Go Playground and this: []string{"art", "football"}:

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    // ********************* Marshal *********************
    u := map[string]interface{}{}
    u["name"] = "kish"
    u["age"] = 28
    u["work"] = "engine"
    //u["hobbies"] = []string{"art", "football"}
    u["hobbies"] = "art"

    b, err := json.Marshal(u)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(b))

    // ********************* Unmarshal *********************
    var a interface{}
    err = json.Unmarshal(b, &a)
    if err != nil {
        fmt.Println("error:", err)
    }
    fmt.Println(a)
}

output:

{"age":28,"hobbies":"art","name":"kish","work":"engine"}
map[name:kish work:engine age:28 hobbies:art]

You want to Unmarshal it, so try this simple working example ([]byte(kpi.(string))):

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    var kpi interface{} = st
    var a []Animal
    err := json.Unmarshal([]byte(kpi.(string)), &a)
    if err != nil {
        fmt.Println("error:", err)
    }
    fmt.Println(a)
}

type Animal struct {
    Name  string
    Order string
}

var st = `[
    {"Name": "Platypus", "Order": "Monotremata"},
    {"Name": "Quoll",    "Order": "Dasyuromorphia"}
]`

output:

[{Platypus Monotremata} {Quoll Dasyuromorphia}]

Working example using ([]byte(*kpi.(*string))):

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    var kpi interface{} = &st
    var a []Animal
    err := json.Unmarshal([]byte(*kpi.(*string)), &a)
    if err != nil {
        fmt.Println("error:", err)
    }
    fmt.Println(a)
}

type Animal struct {
    Name  string
    Order string
}

var st = `[
    {"Name": "Platypus", "Order": "Monotremata"},
    {"Name": "Quoll",    "Order": "Dasyuromorphia"}
]`

Marshal:

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    u := map[string]interface{}{}
    u["1"] = "one"
    b, err := json.Marshal(u)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(b))
}

output:

{"1":"one"}

I hope this helps.