Are you finding optix.51.dll just press download button to get your optix.51.dll file.
If some software in your windows OS wants optix.51.dll your operating system will ask for DLL file. When that DLL file is not available you will get this error.
If you have moved your file or changed the name of dll file you will get this error.
You will mostly see there errors when you dll file is missing:
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