WorkItem Tracking error when performing clone operation in Azure Devops Today I received an error when trying to clone test plans in Azure DevOps web portal. 
Steps:


In Visual Studio, I select the Test Case option
I find the Test Plan I want to clone, select it , and then select the "Clone test plan" button.
In the "Clone Test Plan" window, accept all defaults and click Clone.


Result: an error is received: "Clone operation encountered a WorkItem Tracking error while processing test case xxxx. Retry the operation. Error details: TF51535: Cannot find field 25."

It is extremely unfortunate that this issue happened today while I am trying to clone a large test plan. I will have to manually copy the hundreds of tests in order to have the new test plan ready for Monday morning.
This is an indication that someone removed a field from work item configuration that was represented by 25.  It's more of an issue of clean-up not being done when fields are removed.  See more about this error on Microsoft's Developer Community. 

This other item in the community suggests a different problem when using a particular extension and a fix has been supplied.