How do I just electronically file the fed return and mail in the state in April. Your system is not letting me do that?

You can electronically file the Federal return and mail in the state return later. 

I would note that if you owe the state money, the money is typically due April 15th whether you e-file in January or mail the return in April. You can choose to pay via credit card or bank at a later date or choose to mail a check in later. 

Here are two ways to skip your state in TurboTax Online:

1. Delete your state return, file your federal return now, and then come back later re-do your state return
2. Pay for both your federal and state returns now and when you get to the Here are your filing options screen, select Change next to your state return. On the Change filing method screen, select I'm not ready to file and Save.

Not sure what version of TurboTax your using. I use TT Premier currently, but I have used Basic and Deluxe for years. I always file my Federal Taxes electronically and mail my State return at a later date. When you get to the FILE tab/ FILE a RETURN the first option asked "Is do you want to e-file your federal return for free? Choose E-file. Then select CONTINUE. This page asks if you want to E-file your State Return or Do NOT e-file my State Return I'll file it later. Choose the latter. Then continue to on and Print copy of to mail at you pleasure. Good Luck.