Is the folder you are trying to change security for the root level folder of the briefcase? Or is it a sub-folder (or sub-sub-folder, etc.)?
I would suggest checking that your user has permissions to Modify at the parent folder level of whichever folder you are wanting to make changes to.
You can also check out this KB article we have on enforcing Briefcase permissions which might help out: https://solutions.teamdynamix.com/TDClient/KB/ArticleDet?ID=8650
Please let me know if you have any further questions.