To maximize my solo 401K contribution, I get a box saying that only part of it is deductible. Not sure if I should use the max amount or the full deductible amount?

I am a sole proprietorship with a solo 401K., no employees.  When I ask TurboTax to calculate the max solo 401K contribution, it gives me a total of $36,000.  It then says that only $24,000 is deductible.  Not sure which amount to use now, the max contribution number or the max deductible number.  Obviously, the higher contribution results in lower overall tax, but I just want to make sure I'm doing this correctly and not missing anything important.  Thanks.