If this is related to Form M1PR - You may file and claim a refund for up to one year after the original due date of August 15th. If you file later, you won’t receive a refund.
You can't file a previous year M1PR return through TurboTax. Instead, file directly through the Minnesota Department of Revenue.
For 2015: http://www.revenue.state.mn.us/Forms_and_Instructions/m1pr_15_fillin.pdf