How to resolve “The item you were attempting to purchase could not be found”

If the app is not published, then the account you are using must be declared as a test account, check it here:

Setting Test accounts.

Make sure you have done all these steps:

• The versionCode of the app you're running has to be the same as the apk you have loaded in the Google Play Console.
• The account you are using for testing is not the same account you are using to public the app in the play store;
• The account you are using for testing has been declared as a test account. If not set your account as a tester;
• Your app has to be published in the Play Store. If you are testing your app you probably want to publish it in the Internal testing section. If your app is "In review", the app has not been published yet, hence in-app purchases won't work.;
• If your app has just been published it might take a few hours to make In-app purchases work;
• If you have just created an In-app product, you might have to wait a few hours before it actually works.

You can find the complete tutorial on how to implement In-App Purchases here

This error is telling you that you didn't add the item on Google play developer console under in-app product

Steps to add item

1. Login with Developer account in Google Play Developer Console.
2. Select Your App.
3. From left side Select In-app Products
4. Click on "+ Add new product" button
5. Add new Product window in open. Now enter product id and select Item Type
6. Click on "Continue"
7. Now new window is open. Now enter Title,Description,Price
8. Click on Save or Activate button

Note : If you click on Save button then the item is only saved in your in-app products and it's still not active to use. you have to manually activate the item and wait for 2 or 3 hours to be active.

Most important : If you want to activate your item immediately then you have to click on Activate button at step 8. it will activate your item immediately and save it automatically.