If you opened the link from the first post, in the description of the extension you can read:
Note: Starting on August 2, 2022, all new apps must use Billing Library version 4 or newer. By November 1, 2022, all updates to existing apps must use Billing Library version 4 or newer.
This billing extension will not be updated anymore. You have the following options to update your app:
- Use an App Inventor distribution, which offers a billing component like Kodular or Niotron
- Use the free billing extension by oseamiya, which supports the Google Billing API Version 4.
- Use the paid billing extension by JEWEL (12 USD), which supports the Google Billing API Version 5.
- Use the paid billing extension by Xoma (15 USD), which supports the Google Billing API Version 5.