Something went wrong while saving this configuration: Area is already set

This is a known issue in magento 2.2.4. It can be fixed by modifying the function setForcedArea() in Magento\Email\Model\AbstractTemplate.php

Update

public function setForcedArea($templateId)
{ if ($this->area) {
throw new \LogicException(__('Area is already set'));
}
$this->area = $this->emailConfig->getTemplateArea($templateId);
return $this;
}

To

public function setForcedArea($templateId)
{ if (!isset($this->area)) {
$this->area = $this->emailConfig->getTemplateArea($templateId);
}
return $this;
}