I don't use that tool very often, but I see what you mean. I just tried it with Elements 2020 and 2021 and neither allowed me to adjust the pressure.
Has it ever worked for you? If so, you can try resetting the preferences file. Go to Edit>Preferences>General tab and click the Reset Preferences on next launch button. I don't know of any other tricks, and the HelpX article suggests that it should work, even if you don't have a stylus pad.
Added: You can't change the pressure for the Warp tool either, but I can change it for all other brush tools in Liquify. Perhaps it is working as designed.