Wordpress update or install with ftp: can't create directory xxx

In short, wordpress uses ftp to manipulate the local filesystem, and the FTP user can not create the dir, even if you used some particular user. Look at your FTP server configuration, likely it's using some special user regardless of the login used for connection.

For example, on my vsftpd, this involves putting

write_enable=YES

which is not the default.

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