Make sure that box 9a shows completely blank in TurboTax - do not leave even a "0" or a space in the box.  

The program reads even a 0 as a value, so it will give a percentage error.  If the box appears to be blank and is still giving this error, then try clicking in the box and deleting any potential spaces.  If this fails and you imported your 1099-R, then you should delete the import and enter the values manually as there was likely an issue with the importing of your form.