Apparently Google Chrome sometimes mess up the google account that Chrome itself has been signed in at chrome://settings/, and the accounts you signed in with the browser tab.
That could give you an error when you try to purchase the extension license.
This article talks about some simple steps to straighten it up.
However there are several things that might prevent you from purchasing license via google payment so make sure those cases do not apply to you first, before trying those steps.
First, please read this article as instructions below contain risk of losing your extension data.
If your Chrome is already signed in with your google account,
- Disconnect it from the google account at chrome://settings/.
- Reboot your Chrome browser.
- Go to License tab in the extension's option page.
- Click Purchase button - this should open google log in page in new tab
- Log in with your google account.
If your Chrome is not signed in with any google account yet,
- Get your Chrome signed in your google account at chrome://settings/.
- Go to License tab in the extension's option page.
- Click Purchase button - this should open google payment window.