I was doing an exe upgrade from 2013 to 2015 for a retail customer. They wanted Signature capture for drivers who are delivering goods to customer and they don't want them with a lot of paper. The solution I suggested it to use web client. Unfortunately 2013 does not allow addins to be run on the web client. So it was either doing a full upgrade or just exe upgrade. They decided with just exe upgrade.
While doing the upgrade I ran into the above error during synchronization of tables.
"A table key includes a field that cannot be used as a key: table Transaction Header, key Included in Statistics, field Included in Statistics."
The error confused me with Statistics sql fields on sql. As well as everything worked fine in 2013.
After looking in detail I found that the LS Retail had "Included in Statistics" field as a flowfield and there was a key but it was disabled.
After deleting the key, I was able to finish the executable upgrade.