Looks like WordPress Toolkit unable to dump database of your source WordPress instance using access credentials, retrieved from the wp-config.php of this instance. The initial issue EXTWPTOOLK-636 fixed in WordPress 2.2.0 (
WordPress Toolkit - Plesk Extensions):
[-] If cloning of a WordPress installation was blocked by database import or export errors, the WordPress Toolkit reported that the installation was not configured, but did not explain the actual problem. Now it correctly detects and reports the problem that caused the error. (EXTWPTOOLK-636)
So, at now you get an actual error which block the cloning process. Could you please check ability to connect to database of your source WordPress manually with credentials, specified in wp-config.php?