I didn't find the pKa prediction in the "Show Chemical Properties" Window. I have a license for ChemDraw Professional for Mac.
Currently prediction of pKa value through "Show Chemical Properties Window" available only in ChemDraw Professional for Windows.
So it is not possible to get that function in the Mac environment.
If you have a Windows Operating System, then download the Windows version of ChemDraw Professional and use that product version to predict the pKa value.