I filled out a form 8888, checked off box 35a on 1040, but i got a check instead of an i-bond. Why?

Only the IRS could explain that to you.  TurboTax gets no information from the IRS after you file your tax return.

If the Form 8888 was included with your tax return, then your question can only be answered by the IRS.

Call the IRS: 1-800-829-1040 hours 7 AM - 7 PM local time Monday-Friday
When calling the IRS do NOT choose the first option re: "Refund", or it will send you to an automated phone line.
So after first choosing your language, then do NOT choose Option 1 (refund info). Choose option 2 for "personal income tax" instead.
Then press 1 for "form, tax history, or payment".
Then press 3 "for all other questions."
Then press 2 "for all other questions."
- When it asks you to enter your SSN or EIN to access your account information, don't enter anything.
- After it asks twice, you will get another menu.

Press 2 for personal or individual tax questions.
Then press 3 for all other inquiries
It should then transfer you to an agent.

sometimes the IRS messes up. And if you're asking if you can send back your refund to have it applied to Ibonds the answer is no.