TurboTax 2018 requires Mac OS X 10.12 of higher.
You can still use the web based online version but the pricing is different. You will have to ask customer support if you can switch since you did not purchase form TurboTax - you might have to return it to Costco.
Here is a TurboTax FAQ for contacting customer support.
https://ttlc.intuit.com/questions/1899263-what-is-the-turbotax-phone-number
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
