If you are getting this kind of error while opening Book Keeper, please follow the below mentioned points:
Make sure you are on the latest version of the application.
Go to Control Panel > Uninstall a Program > Check if there are two versions available of BookKeeper. If yes, then uninstall the application with version 7.6.2. Do not uninstall the 7.9 series application.
Go to My Files/File Manager > C Drive > Users > Click on Your PC username > AppData > Local.
Here, find the folder with name Just_Apps. Rename this folder to Just_Apps_OLD.
Now reopen BookKeeper.
If the issue still persist, or you are confused about the steps, then please contact BookKeeper Support Team.