_test.go file can't find a function defined in another go file in the same directory

Why the compiler shows that the function newDeck() is undeclared in deck_test.go knowing that it's already declared in deck.go and it's working well? If you could help me that will be great thank you.

deck.go

package main

import (
    "fmt"
    "strings"
)

type deck []string

func (d deck) print() {
    for i, card := range d {
        fmt.Println(i, card)
    }
}

func newDeck() deck {
    cards := deck{}

cardSuits := []string{"Spades", "Diamonds", "Hearts", "Clubs"}
cardValue := []string{"Ace", "Two", "Three", "Four"}

for _, suit := range cardSuits {
    for _, value := range cardValue {
        cards = append(cards, suit+" of "+value)
    }
}

return cards
}

deck_test.go

package main

import (
    "testing"
)

func TestNewDeck(t *testing.T) {
    d := newDeck()

    if len(d) != 16 {
        t.Error("Expected deck length of 16, but got", len(d))
    }

}

Image showing the problem