错误消息告诉您不能有以下内容:

The error message is telling you that you can't have the following:

<%= form_for nil do |f| %>
<%= form_for [] do |f| %>

我的猜测是您的 @contact 设置为 nil 并且它不是来自您的 Contact#new 操作.

My guess here is that your @contact is set to nil and that it doesn't come from your Contact#new action.

仅供参考，如果您这样做，它会很简单:

FYI it would simply work if you do this:

<%= form_for Contact.new do |f| %>

虽然不推荐.

您需要检查包含您的表单的视图实际上是由您的 ContactsController 的 new 操作呈现的.

You need to check that the view containing your form is actually rendered by the new action of your ContactsController.