When I try to create a user for the first time, why do I see this error?

Looks like there was an error on sending the password mail. Please contact the admin!

When using DokuWiki's ACL feature, it uses PHP's mail function to send the password email. You will get this error when that call fails. This usually means the 'sendmail' command is not available, and you need to install sendmail or some other MTA which mimics the sendmail command. On Unix systems this should work out of the box, on Windows systems you will need to configure the access to your MTA in the php.ini.

When configuring PHP for sending mails is not possible for some reason, you can use the swiftmail plugin to configure an external mail server.

If you don't get an error message but still the mail never arrives, check the log files on the sending and the receiving mail servers. Also make sure the mails are not eaten by a spam filter.