Why do I get this error: Could not instantiate mail function?

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Well,this can happen due to following reasons;

PHP function mail() has been disabled or not working;
You are running an older version on PHP in your server;
The Sendmail Path is wrong (usually it is not the case, bacause it is typically filled in by Joomla! during the initial setup;
You are on a local environment and don't have a mail server installed.

Here are the steps to solve it:

Go to Admin panel > Global Configuration > Server > Mail Settings > Mailer and select SENDMAIL there,instead of PHP MAIL FUNCTION.
If the above point doesn't solve your issue,then the best thing is to contact your webhost and ask them to rectify the PHP Mail Function in the PHP in their server and also ask them to upgrade the PHP to the latest version, OR better yet is to use SMTP instead of PHP Mail function or Sendmail.
You can also try to change the contact email address and remove the link to a user, for some reason this solves this issue sometimes;
Please read this other article: Why emails are not being sent?