Problem 

In a Test case the Test Details custom fields are not shown and instead it shows the error message below:

Solution

1. Go to the Project Settings, Screens and edit the screens
2. Choose the Screen Scheme for the Test issue type
3. Edit the default screen of the scheme and make sure that it has the following configuration:
   
   

Solution 2

1. Go to your project -> Project Settings -> Issue types
   
2. Button Actions -> Edit issue types
   
   
   
   
3. Remove all the Xray Issue Types -> Save
   
   
   
   
4. Go back to your Project -> Project Settings -> Button Actions - Add Xray Issue Types
   
   
   1. If you don't see the option "Add Xray Issue Types", make sure that the user belongs to a group that is allowed to "Add Xray Issue Types". Check this on "Miscellaneous" menu of Xray configuration.
      
      

Link for related documentation

Custom Fields and Screen Configuration