Hi,
You just select the folder :
C:\Users\Public\Documents\National Instruments\TestStand 2021 (32-bit)\UserInterfaces\Simple
It can be a simple of full featured UI - whichever you are uisng.
Further there are different UI - VB,VC,LV etc.You can choose with whichever you are testing and copy the whole folder content as part of your deployment files.
One a seperate note did you check the TS Deployment tool which does the same.
In any case you need to add this folder to your deployment.
Ravi