I don't have the TurboTax software for Missouri, so I can't test this, but I have a feeling that the root of the problem is the Missouri instructions for an amended return. The instructions for "Reason for Amending Your Return" (on page 10 of the 2018 instructions for Form MO-1040) don't make any provision for filing an amended state return without an amended federal return. Although the fourth choice on the form itself just says "Correction other than A, B, or C," the instructions define the fourth choice as "filing an amended Missouri return as a result of filing an amended federal return." TurboTax is just doing what the state instructions say.

However, on Form MO-1040 the space for the date of the federal amended return says "Enter date of federal amended return, if filed" [emphasis added]. So it seems like it's okay to simply omit the date if you are not filing a federal amended return. You should be able to ignore the error in TurboTax. When you get the error in the final review, just click Continue without entering anything. You wouldn't be able to e-file, but the amended return can't be e-filed anyway. It has to be printed and filed by mail.

@robtm  Can you add anything to this?