The error message displayed implies that the test harness in question is corrupted, and it is unlikely that anything can be recovered from it. This can happen when something goes wrong while saving .slx files, but it is hard to tell what corrupted the harness. As per my understanding, if this is the situation, you will not be able to recover the harness. You will need to delete it and create a new one.
If you saved the harness in a previous release and are now trying to open it in a later release, I would suggest you try opening this in the same release where it was saved. This may be similar to the issue discussed in the following MATLAB answer thread: https://www.mathworks.com/matlabcentral/answers/767567-why-do-i-get-an-error-when-opening-my-stateflow-diagram-which-says-file-does-not-contain-the-reques
You can contact the MathWorks Technical Support team by accessing the following link, as they will be able to provide an accurate resolution to this issue: https://www.mathworks.com/support/contact_us.html
