Hi Manish , AssistEdit is Internal function in Sales Header, you can only use function which declared as "External" in your extension.
You may need to copy the same function to your custom codeunit instead of calling from standard object.

If you are working on-premise you can change "Target" to Internal in your app.json to be able to use standard internal function in your extension while you change your solution to custom codeunit for those code calling standard internal function.