hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)

edit

This is my code:

#creating our own dataset
import cv2
import numpy as np
import os
import sys

image_x, image_y = 50, 50

def create_folder(dir_name):
    if not os.path.exists('D:/Pics'):
        os.mkdir('D:/Pics')
        print("Directory", dir_name ,"created")
    else:
        print("Directory", dir_name ,"already exists")

def store_images(g_id):
    total_pics = 1200
    cap = cv2.VideoCapture(0)
    x, y, w, h = 300, 50, 350, 350
    create_folder('D:/Pics/' + str(g_id))
    pic_no = 0
    flag_start_capturing = False
    frames = 0

    while True:
        ret, frame = cap.read()
        frame = cv2.flip(frame,1)
        hsv = cv2.cvtColor(frame, cv2.COLOR_BGR2HSV)
        mask2 = cv2.inRange(hsv, np.array([2,50,60]), np.array([125,150,255]))
        res = cv2.bitwise_and(frame, frame, mask=mask2)
        gray = cv2.cvtColor(res,cv2.COLOR_BGR2GRAY)
        median = cv2.GaussianBlur(gray, (5,5), 0)

        kernal_square = np.ones((5,5), np.uint8)
        dilation = cv2.dilate(median, kernal_square, iterations=2)
        opening = cv2.morphologyEx(dilation, cv2.MORPH_CLOSE, kernal_square)

        ret, thresh = cv2.threshold(opening, 30, 255, cv2.THRESH_BINARY)
        thresh = thresh[y:y + h, x:x +w]
        contours, hierarchy = cv2.findContours(thresh.copy(), cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)

        if len(contours) > 0:
            contour = max(contours, key=cv2.contourArea)
            if cv2.contourArea(contour) > 10000 and frames > 50:
                x1, y1, w1, h1 = cv2.boundingRect(contour)
                pic_no += 1
                save_img = thresh[y1:y1 + h1, x1:x1 + w1]
                save_img = cv2.resize(save_img, (image_x, image_y))
                cv2.putText(frame, "capturing...", (5, 100), cv2.FONT_HERSHEY_TRIPLEX, 2, (127, 127, 255))
                status = cv2.imwrite(os.path.join('D:/Pics/gestures/'+ str(g_id) + '/' + str(pic_no) + '.jpg'), save_img)
                print("Image written to file system: ",status)

            cv2.rectangle(frame, (x,y), (x+w, y+h), (0, 255, 0), 2)
            cv2.putText(frame, str(pic_no), (30, 400), cv2.FONT_HERSHEY_TRIPLEX, 1.5, (127, 127, 255))
            cv2.imshow("Capturing gesture", frame)
            cv2.imshow("Thresh", thresh)
            keypress = cv2.waitKey(1)
            if keypress == ord('c'):
                if flag_start_capturing == False:
                    flag_start_capturing = True
                else: 
                    flag_start_capturing = False
                    frames = 0

            if flag_start_capturing == True:
                frames += 1
            if pic_no == total_pics:
                break

g_id = input("Enter the gesture no: ")
store_images(g_id)

Error:

 cv2.error: OpenCV(4.2.0) C:\projects\opencv-python\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'