Where are you opening 'software update'? I don't have an S21 (moto z4) so I cannot replicate your steps. Are you able to install/update other apps from the Play Store? Have you looked at this link from another related thread:
Hi, when I try to open software update I get "software update keeps stopping." I cannot update software. Yes I have tried clearing cache and all the other basic possible fixes. Please help. Thanks
...Just another VZW customer...trying to offer some assistance...