While processing a workflow step, it is getting into a failure and the state is failed:
 

System.IO.FileNotFoundException: Could not load file or assembly 'PostSharp, Version=4.2.20.0, Culture=neutral, PublicKeyToken=b13fd38b8f9c99d7' or one of its dependencies.

 

Note: This issue occurs for Workflow created before Tridion Sites 9.5

 

Solution: Recompile External Activities against Sites Tridion 10.0 APIs.

The Content Manager tracing facility in older versions (before Tridion Sites 9.5) used the PostSharp library to inject the tracing code into the Intermediate Language (IL).
In fact, this was removed in Sites 9.5 resulting in mostly dysfunctional Content Manager tracing, instead this has been replaced with an alternative (called Fody) in Tridion Sites 9.6.