For most access denied errors there's usually a rights/permissions issue. First, I would retry your existing command with an elevated powershell instance. To elevate, right click on the powershell icon and choose run as administrator.
If that fails, navigate to the location with your user account and verify that you have permissions (for the purpose of testing, ensure you have "full access" to rule out permissions). Re-mediate any permissions issues and try again.
If you're still having issues consider whether or not you're on a domain, whether or not the files are on a network/drive share, and whether or not that share is linux or microsoft. It might be helpful to update your question with those specifics.