Before you can use case feed in Lightning Experience, you must recreate the Log a Call publisher as a quick action.
|Available in: Lightning Experience
|To set up cases for Lightning Experience:||“Manage Cases”|
From the object management settings for cases, go to Buttons, Links, and Actions.
Click New Action.
Under Action Type, select Log a Call.
Under Standard Label Type, select Log a Call.
To choose the fields users see, customize the action’s layout, then click Save.
On case page layouts, drag the new quick action to the Salesforce1 and Lightning Experience Actions section, then click Save.