When activating Classic Commerce, some new SQL tables are added in your database. But this process can fail sometimes if the ClassicPress SQL tables prefix is too long: the table name can’t contain more than 64 characters.
This can cause a known issue where a customer who purchases a downloadable product finds the Complete Order Email doesn’t contain any download link for the purchased products.
The ClassicPress SQL Table prefix is defined during ClassicPress install, and is stored in the file called wp-config.php.
How to solve the issue?
To solve this issue, there’s only one solution: renaming the ClassicPress SQL table prefix.
You can search for a plugin to do this for you, or you can do it manually.
If you want to do it yourself, you need to rename all tables names using a tool like phpMyAdmin, reduce the table’s prefix, and update the prefix value in wp-config.php. When you’ve done this, deactivate Classic Commerce and re-activate it. Don’t worry you won’t lose any data, and re-enabling Classic Commerce should force it to create missing SQL tables.
Please create a backup of your database and of your site before doing such actions.