wp_mail is not sending email if I pass an array of emails

If you want to send the email to more than one user, then you can write a loop.

foreach ($employee_emails as $email) {
    wp_mail( $email, $subject, $message );
}

This will loop through all the email addresses in the array and send the email to each one of them.

UPDATE

You can store your email addresses into a single string, separated by commas:

$employee_emails="";
foreach ( $employee_ids_to_notify as $employee ) {
    $employee_emails .= get_the_author_meta( 'email', $employee ).', ';
}

Then you can pass it as a single string to wp_mail.