A came letter from the MN Dep of Rev. saying they have not received my sales & use tax return. Wouldn't this have been taken care of when I did my taxes through TurboTax?

Depends on what the letter is referring to:

1)  IF you don't run a business where you sell stuff, then it's probably referring to paying sales/use tax for buying a lot of stuff online for personal use that you didn't pay those MN sales taxes for.  States are attempting to crack down on these sales, with various rates of success, and most retailers do asses state sales taxes, even with Online purchases.  There is a section during the MN state interview that takes care of that...but only if you actually fill it out.  So if MN now has a way to track your purchases from out of state, perhaps they are dunning you for those now.
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Or..maybe it's this:

2)  But if you run a business where you sell a lot of stuff, or sell a lot of stuff online, then those MN sales taxes that you are supposed to be collecting and remitting to MN, are reported and paid to MN entirely separately from the TTX software. Those forms and reports and payments are only done thru MN's separate filing&payment system. In that case you would need to read the following....and maybe get help from a local CPA:

www.revenue.state.mn.us/forms_and_instructions/sales_tax_booklet.pdf
and
http://www.revenue.state.mn.us/businesses/sut/Pages/Filing_Due_Dates.aspx
and
http://www.revenue.state.mn.us/businesses/Pages/Tax-Types.aspx