There is an error in the instructions inside Pro Series 2021 for Client Data Conversion from UltraTaxCS when converting from tax year 2020 to 2021.  

This instruction has an error....

11.An additional file is also needed from the data folder. Open the UltraTax install directory (e.g., C:\WinCSI) and navigate to the UT19DATA folder.
11.1 Find the file CltList.WT9, right-click on it and choose Copy.

"UT19DATA"  should be "UT20DATA" ...and... "CltList.WT9"  should be "CltList.WT0"

The conversion won't work unless you get the correct file for the 2020 tax year.