JoleneP You’re running into this error because the proc_open() function is disabled on your server.
Flarum (via SwiftMailer) tries to use sendmail, which relies on proc_open(). Since your hosting environment has this function disabled (quite common on shared hosting), the mailer fails.
How to fix it:
The easiest solution is to switch your mail driver from sendmail to SMTP in the Flarum admin panel.
Use an SMTP provider (for example: Gmail SMTP, Mailgun, SendGrid, etc.).
After saving the SMTP settings, test sending an email again.
Alternative (not recommended on shared hosting):
Ask your hosting provider to enable proc_open() — but most providers won’t allow this for security reasons.
Summary:
This is not a Flarum bug, but a server limitation. Switching to SMTP will resolve the issue.
