Переписал функции и выдает 500 ошибку теперь из-за нее, но причину не указывает:
func searchHandler(w http.ResponseWriter, r *http.Request) {
    u, err := url.Parse(r.URL.String())
    if err != nil {
        w.WriteHeader(http.StatusInternalServerError)
        w.Write([]byte("Internal server error"))
        return
    }
    params := u.Query()
    searchKey := params.Get("q")
    page := params.Get("page")
    if page == "" {
        page = "1"
    }
    search := &Search{}
    search.SearchKey = searchKey
    next, err := strconv.Atoi(page)
    if err != nil {
        http.Error(w, "Unexpected server error", http.StatusInternalServerError)
        return
    }
    search.NextPage = next
    pageSize := 20
    endpoint := fmt.Sprintf("https://newsapi.org/v2/everything?q=%s&pageSize=%d&page=%d&apiKey=%s&sortBy=publishedAt&language=en", url.QueryEscape(search.SearchKey), pageSize, search.NextPage, *apiKey)
    resp, err := http.Get(endpoint)
    if err != nil {
        w.WriteHeader(http.StatusInternalServerError)
        return
    }
    defer resp.Body.Close()
    if resp.StatusCode != 200 {
        w.WriteHeader(http.StatusInternalServerError)
        return
    }
    err = json.NewDecoder(resp.Body).Decode(&search.Results)
    if err != nil {
        w.WriteHeader(http.StatusInternalServerError)
        return
    }
    search.TotalPages = int(math.Ceil(float64(search.Results.TotalResults / pageSize)))
    err = tpl.Execute(w, search)
    if err != nil {
        w.WriteHeader(http.StatusInternalServerError)
    }
}