If you get error "Can't find destination folder" during publication, it means that website builder could not find folder on your hosting where to publish website files.

When using "Custom (API)" method:
To fix this error you need to review parameters that you pass to website builder API when creating session for a customer. Parameter responsible for this error is uploadDir. Make sure that it has a value of relative directory on your hosting in respect to FTP account that you pass along with other parameters.
For example, if customer "A" on your server has web folder "/home/user_A/public_html" and FTP account (parameters "username", "password", "apiUrl") that you pass to website builder API points to directory "/home/user_A", then parameter "uploadDir" must have value "public_html".

When using pre-defined hosting panel (cPanel, Plesk, etc.):
Website builder plugin on hosting control panel should automatically detect FTP account and customer's web folder where to publish website. However, in rare cases API of hosting panels can return wrong FTP folder and, as a result, publication ends with error. In this case you may try to reset FTP account for customer on your hosting control panel. If this does not help, then open a ticket in support system and describe problem — our technicians will provide a solution.