Please follow these steps to change your Google account for in-app purchases:
- Add the account you need to your device (Settings -> Google -> Account -> Add account)
- Delete the app.
- Go to the browser version of the Google Play store.
- Check the Account tab in the Google Play menu to make sure you've logged into the right Google account.
- Find the app and install it.
- Open the app and try making the purchase again.
You can also read this article on how to add Google accounts to your device.