If this is the first time your sons have filed a return with the CRA, they will likely not be able to NETFILE and will have to print & mail their returns.
If this is not the first time they have filed, please review your sons’ NETFILE eligibility here: https://www.canada.ca/en/revenue-agency/services/e-services/e-services-individuals/netfile-overview/eligibility.html
Also, if you entered a NETFILE Access Code (NAC), please double check that it is correct and has all 8 characters. You can also try submitting your return without the NAC. The NAC is optional and not required to file your return.
If you are still unable to resolve this issue, please contact us by phone. To talk to our phone support team, visit https://support.turbotax.intuit.ca/contact/. After entering some basic info about what you need help with, the page will give you the phone number for the team best suited to help.