TT states I cannot make a non-deductible IRA contribution

Hello,

I am using Turbo Tax Home and Business. I am self employed and have a Solo 401k plan. I have no employees and file as sgl. I made the max contribution to my Solo 401k plan. 

However that should not preclude me from also making a non-deductible contribution to an IRA. TT is stating that I cannot do so. That I am ineligible. 

There is no IRS rule that states that I cannot make a non-deductible contribution. And TT does in fact ask if one has made any non-deductible contributions and if one wants to trace them. Can anyone explain why TT may not be allowing for it this year?