Why is Turbo Tax recommending I lose $600 refund because it changes the state refund when I enter a tax credit that has nothing to do with state?

As you enter information into TurboTax, it will automatically calculate what is the best deduction for your tax situation. If the program switched from itemized deductions to the standard deduction, it is because the standard deduction provides you with greatest benefit. 

It is difficult to determine exactly what happened without knowing what credit you were entering, but perhaps the credit reduced or changed your itemized deductions so that it was a greater deduction to claim the standard deduction. 

If you would like to change to claiming itemized deductions instead of the standard deduction, please see the instructions in this TurboTax Article.