If you are on WordPress 5.8 or above and you want to place Ads using widgets follow these steps:
Head over to WordPress Dashboard>Apperence>Widgets
Click on the add widget button and search for AdCenter
You can then select any widget like a Single Ad, Random Ad, etc.
Then select the ad or Ad group you want to place.
You can then select the number of columns you want and the devices you want to show the ads on.
And Click on Update