参考地址：https://help.salesforce.com/articleView?err=1&id=dev_tabdef.htm&type=5

Lightning Component Tabs

Lightning component tabs make Lightning components available in the Salesforce mobile app (via the navigation list) or in Lightning Experience. Lightning components aren’t supported in Salesforce Classic.

Note

To expose Lightning components via Lightning component tabs, you must enable and deploy My Domain.

大意就是，

1.lightning不支持Salesforce Classic，要切换到lightning experience(点击头像切换)。

2.enable and deploy My Domain。发布你的domain，如下：

click setup, search domain,regist a domain.当你的domian注册成功后，你会收到一个邮件，然后你就可以添加tab了