You need to clear SharePoint Designer 2013 cache. To clear the cache you need to execute the following command from the command line as administrator on your PC and reopen SharePoint Designer:
rmdir "%LOCALAPPDATA%\Microsoft\WebsiteCache\" /s /q
rmdir "%APPDATA%\Microsoft\SharePoint Designer\ProxyAssemblyCache\" /s /q
Or you can download and execute the bat file.
After clearing the cache and running SharePoint Designer you may need to close and open SharePoint Designer one more time, then the error should disappear.
I also recommend installing recent updates for SharePoint Designer 2013. This will help to avoid annoying error when configuring workflows.
Note: for OnPremises version only.
If you use SharePoint Designer on the same server where has installed Workflow Manager and see the message Server-side activities have been updated, please check the link.