I don't know what the .NET system is that you have. You're going to need an intermediary system/service (perhaps one that you build), that can communicate to both the .NET system and the Magento REST API.
If it's a one-off process, you can perhaps do a export from the database and create a file feed to read in and submit to Magento via Rest API.
Alternatively, does the .NET system have an API which you can communicate with and then your system can just be the translator of messages from one system to another.
How should I solve the below mentioned error?
s:1147:"Fatal Error: 'Uncaught Error: Call to a member function clearMediaAttribute() on null in /ProductRepository.php:532 Stack trace: #0 /ProductRepository.php(592): \ProductRepository->processMediaGallery(Object(Magento\Catalog\Model\Product\Interceptor), Array) #1 [internal function]: \ProductRepository->save(Object(\Product\Interceptor), false) #2 /Interceptor.php(74): call_user_func_array(Array, Array) #3 /Chain.php(70): \ProductRepository\Interceptor->___callParent('save', Array) #4 /Chain.php(63): \Chain->invokeNext('Magento\\Catalog...', 'save', Object' in '/ProductRepository.php' on line 532";
Sorry, I've not seen that error before.