I have an iMacpurchased late 2009 , version 10.13.6 is there a way to upgrade to at least Mojave version 10.14 the Turbo Tax will not work as it is

A late 2009 iMac cannot be upgraded beyond macOS 10.13.6.

You may want to consider using the online version of TurboTax. It will run on 10.13.6 and also versions older than that. See: https://turbotax.intuit.com/personal-taxes/online/system-requirements

FWIW, Intuit does have an online version which may suffice.

Have you considered replacing the computer with a more up-to-date Mac? You have many alternatives available to you in all price ranges.

I had the same problem with my 2011 MacBook Pro. I ended up using the online version of TurboTax.