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Possibly you have an old version of plugin installed on cPanel server. It does not include functionality of automatic adding FTP accounts if needed for publication. Update the plugin by instruction. This should fix the problem.
Problem occurs when site website builder cannot create FTP account when opening website builder from cPanel. To fix this problem you need to ensure that you have not exceeded limit of FTP accounts in your cPanel account. Delete unused FTP accounts. There may be old FTP accounts left that were created by website builder and not used anymore (they are of type "a1b2c3d4@yoursite.com"). As soon as you have space for at least one FTP account the website publication should work.
Make sure that "FTP Manager" is enabled on WHM system for your customers. Without it, publication may not always work.