For sendmail to throw that error means it can't figure out what Unix user the web server process is running as. This likely means the user is not in /etc/passwd - perhaps this is a chrooted installation?
If you can't/don't want to figure out the sendmail problem, once you have reset your Wordpress password you could install a plugin to have Wordpress send mails using SMTP instead of by running the sendmail binary. I use a plugin called WP-Mail-SMTP for this.
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