Tax Year Prior to 2020: Can I deduct last years business loss on this years tax return (schedule c)? If so, where/how do I enter it in turbo tax

[KarenL2]

No, you don't deduct the actual Schedule C net loss. However, you may have a Net Operating Loss (NOL) from the prior year (this is rare).  This happens when your business loss, combined with your other income and expenses still results in a loss.  

An NOL can be carried back to the two prior years and applied to those tax returns by amending those tax returns. That said, you can elect to "forego the carryback".  But, you must have made that election on your 2015 tax return.  If you chose that option, you will carry it forward to future to 2016 and future years until it's used up (assuming you had an NOL). 

Here's more information on NOLs that may be helpful to you: IRS Pub 536 Net Operating Loss