See https://www.mathworks.com/matlabcentral/answers/95122-why-do-i-get-license-manager-error-96 for a discription of error -96 .
In R2012a time frame, I think imshow() was still part of the Image Processing Toolbox, and so would require checking for a license for that toolbox to run.
According to that error message, your system is configured to check a license server for additional toolbox licenses. However, the hostname that is listed in the license information is one that name server lookup fails for, such as if the name of it had changed.
The article at https://www.mathworks.com/matlabcentral/answers/99147-where-are-the-license-files-for-matlab-located talks about where the license files are. However, in the case of using a license server, the information might be stored in environment variables; see https://www.mathworks.com/matlabcentral/answers/93105-what-are-the-lm_license_file-and-mlm_license_file-environment-variables-and-how-do-i-set-or-check-fo
You have to locate the place that is directing you to a license server, and change the host name to the correct host name where the license server is... assuming that you should be using a license server and it isn't just a case that your license information is misconfigured.
Or... you could replace the calls to imshow() with calls to image() and possibly colormap() as well
