If I think I qualify for a MN Marriage Credit, but Turbo Tax doesn't let me explore it, how can I override TT?

No, you can't override TurboTax and e-file your tax returns. As xmasbaby0 points out, you probably don't qualify for the credit. 

To qualify for this credit, all of these must be true:

• You are married and filing a joint return
• You and your spouse have taxable earned income, taxable pensions, or taxable Social Security income
• Your joint taxable income is at least $42,000
• The income of the lesser-earning spouse is at least $26,000

If either of you is a nonresident or a part year resident, your income will be pro-rated to include only your Minnesota income. 

In addition, this is a non-refundable credit. That means if your Minnesota tax was $0 before the credit, your credit is reduced to $0. 

If after reviewing these requirements, you still think you qualify, you should review your return and make sure you correctly identified each spouse's income as belonging to them.