A full theme package you buy from marketplace it commonly included: SQL file, Theme
When you install a Magento 2 by importing database from SQL file, you may get the problem related payment method.
That's your website redirected (Status 302) to Cart page after Place Order instead of Success page. i experienced with Smartwave, Plazetheme ...
How to fix this problem:
- Clear your cache by manually deleting the content of /var/cache/ folder , the /var/page-cache/ folder.
- Remove /var/generation/
- Remove /generated/ if you are using Magento 2.2.0
- Reindex your website by using the SSH command line php bin/magento indexer:reindex
- Deploy again the theme by ysubg the SSH command line php bin/magento setup:static-content:deploy
In case you already did all the steps above but the problem is still not resolved, you can use command line composer update to fixed your core Magento 2