I spelled the last name wrong do I need to amend

No, you cannot amend to change your name. 

If your return is accepted by the IRS, do not worry. The IRS was able to cross-reference and identify you based on the rest of your information in your return.  

If your return is rejected by the IRS, you can navigate back to the Personal Info tab and fix the spelling of your name before resubmitting your tax return.  

You can also check your e-file status using the TurboTax E-File Status Lookup Tool.

My last name is speeled Oliveira

So why are they rejecting my signature

Please provide additional details.  It's unusual for the IRS to reject a return because of spelling.  If your return was rejected, what was the message and error code?  You can check your e-file status using the TurboTax E-File Status Lookup Tool.

If your return was rejected by the IRS, you can go back into your return and correct the information:

1. Click My info in the black bar at the left of your screen.
2. Click on Edit next to your name. 
3.  Make the changes and click Continue.
4. Click File in the black bar at the left of your screen to resubmit your return.

@Lele4nnn