I have this use case, where sometimes the code might be partially successful.
Can I return the response in such case were I respond with HTTPS-206, saying,"There was an internal server error in following cases."?
Does 206 code spec, specifies not to mention/ does not allow me to mention internal server error in response?
The HTTP 206 status code (like all status codes in the 2xx range) represents "successful" fulfillment of the request, so if you are trying to represent a partial failure, it would probably not be the best choice. 206 is about the response and generally is in response to a request that uses a range header (in other words, a request for only part of a resource). See https://stackoverflow.com/a/55330328/639520 and https://developer.mozilla.org/en-US/docs/Web/HTTP/Status#206_partial_content for more details.
If there has been an error processing the request, you're best to use a 4xx or 5xx response status code, depending on whether the problem stems from client/request data or something truly internal to the server.