The list of workflow actions on the server references an assembly that does not exist. Some actions will not be available

When I tried to create SharePoint 2010 workflows in SharePoint Designer 2013, I got the following error message shown below:

This error may occur if the system does not have enough memory, I was running a VM at 4GB; when I scale up to 6GB, it’s working fine. in some cases, This error comes up if you don’t have local privileges, therefore, you need to check you local admin privileges. 

If you already checked the previous reasons and the problem not solved! so you need to clear the cache of the following Folders (If didn’t work delete them):

• %USERPROFILE%\AppData\Local\Microsoft\WebsiteCache
• %USERPROFILE%\AppData\Roaming\Microsoft\SharePoint Designer\ProxyAssemblyCache
• %USERPROFILE%\AppData\Roaming\Microsoft\Web Server Extensions\Cache

If the problem still exists that mean someone has extended the Workflow actions available in SPD with a custom action developed in Visual Studio.These custom actions are loaded when your workstation reads the WSS.ACTIONS file on the server.

The file is located in the 14 hive under Template\1033\workflow.Your WSS.Actions file contains a reference to a custom action .dll. Delete WSS.Actions file and your designer should start working again.

Make sure you take a backup of the file first.

Reference: https://mohamedelkassas.wordpress.com/2014/12/26/the-list-of-workflow-actions-on-the-server-references-an-assembly-that-does-not-exist-some-actions-will-not-be-available/