https://forums.freenas.org/index.php?threads/use-root-login-cifs-access-is-denied.21233/

I fixed the issue by changing root’s password via the WebGUI. To do this:
1. Go to the “Accounts” page
2. Click on the “Users” tab
3. Locate and select the user “root”
4. Click on the “Modify” button located at the bottom of the page and a dialogue window will open
5. Enter a password, twice, and click on the Save? button located at the bottom of the dialogue window