If you use a download version you can have the option of admitting a first or last name. But I'm having Computer problems so wondering if the online version allows this. I have not been able to file online however because of this issue so I'm thinking not but you would think that the two would work similarly—you would think….
If you do not have a last name, entering Not Applicable should work. If not you can print and mail your returns.
Unfortunately that will not work. I tried something similar to that and they confused my taxes since it didn't match with my social. IRS has made it clear that I must leave the field blank.
I have printed my taxes for many years. Sad that we have to waste those trees because Intuit —assuming from you— hasn't fixed the problem when it's not a problem with the download version.