If your external is in another server, you have to do a workaround to add the credentials: enter a second svn url for the external on the jenkins job, enter the credentials, and then you can delete the second svn url, as the credentials will be saved!
You have a different solution? A short tip here would help you and many other users who saw this issue last week.
- org.tmatesoft.svn.core.SVNCancelException: svn: No credential to try. Authentication failed at org.tmatesoft.svn.core.internal.wc.SVNErrorManager.cancel(SVNErrorManager.java:36) at org.tmatesoft.svn.core.internal.wc.SVNErrorManager.cancel(SVNErrorManager.java:31)