The error message you are getting is not for the standard deduction vs. itemized deduction. You are entitled on an MFJ return to claim up to $600 when claiming the standard deduction or $300 for Single, Head of Household, or MFS. TurboTax wants you to enter either the $300 or $600.
As for as why it said itemized was better and then gave you the standard, I cannot answer as I cannot see your return. You could go back through the return and see if you can see anything that changed.
