Error in KV decoder, or no KV decoder present

You may want to check the WSJT Yahoo group for support; this is not likely a Flex issue. The KVASD install does require a license agreement, and for Mac OS, some terminal commands. You may need Administrative permissions in Windows and/or installing the software outside the "usual" Program Files folder.

I had the same problem installing WSJT-X on my Flex 1500. After trying everything suggested on the internet, I found the solution. Create an empty file named KVAST.BAT and place it in the in directory where you installed the WSJT-X executable. 

Rick
K3IND

Your solution only tells WSJT-X that there is an executable whereas in my wsjtxin folder lives kvasd.exe. It was downloaded during installation
of WSJT-X 1.5.

In Jim Taylor's words it is

...an implementation of the the Koetter-Vardy algebraic soft-decision decoder for the Reed Solomon (63,12) code....

A word about the Reed–Solomon error correction here:
https://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_error_correction

I have it and run it. To see whether it improves decoding I really should
do a double-blind test with and without it :-)

73, Alex DH2ID