Extension/theme purchases are tied to your Google account, as described in the description of the theme you bought. Log in to your Google account you purchased the theme with and go to the Google Pay activity page (
https://pay.google.com/gp/w/u/2/home/activity). If you actually purchased the theme with that account you should see it there. If you don't see it there, then you probably purchased the theme with other account, so repeat above step for other accounts you own. If you see the theme listed in the activity page, but you can't install it in Chrome Web Store make sure you are logged in the same account in the Chrome Web Store itself. If you still can't install it or you have found duplicate purchases on two different accounts, follow the steps described here
https://support.google.com/chrome_webstore/answer/1060830?hl=en to get a refund.