NoPE is designed to use Burp's CA to sign certificates that it generates. For this to work there is some very simple steps you need perform first.

1. In Burp's Proxy->Options Click 'Import / Export CA certificate'.
2. Select Export Certificate and private key in PKCS12 keystore.
3. Click Next
4. Save it as a file named 'burpca.p12' and store it in the same folder that burp runs out of.
5. Set the password to 'changeit'.

Thats it. You should never have to do that again until you upgrade burp.

Now when you create an non-HTTP listener in NoPE you can select the SSL checkbox before adding it to the listener table. Note: You cannot downgrade an SSL listener to non-SSL if the listener is enabled. You can uncheck 'enable' and the SSL checkbox will be editable again.