Hello,
Apple has blocked the option to record internal audio to avoid unrestricted use of their content, there are third party programs you can use to bypass this but they are not supported by NCH so if it does not work properly there is not much that can be done. One of the programs I have used to bypass this is called ¨Loopback¨
You can also contact NCH Tech support to see if they have anything they can recommend. You can create a ticket for them through the link below:
https://www.nch.com.au/support/index.html 
Good luck!