How to set permissions to allowed for each category and download

[meghaedz]

I want to change permissions of categories and downloads from there permission tab, but I'm getting following message "Error
A user is not allowed to change permissions of their own group(s) parent group(s).". I have created one user group under public and I want to give access to that user  to change permissions for downloads and categories? Thanks in advance. Thank you so much.

[ColinM]

Hi
There is a whole section in the documentation relating to Access and User Group Permissions.
https://www.jdownloads.net/documentation/access-user-group-permissions-v3-9

In Joomla users may be allocated to diferent User Groups, and different User Groups have different  'abilities'.  Also all User Groups, except Public, have a Parent Group.  Child Groups inherit the permissions of their Parent.
At first it can seem quite confusing!! But it is very logical and works well. 
All users belong to at least one user group, namely the Public User Group.  Typically users who log on belong to the Registered and the Public User Groups.

Permissions may only be granted in the backend and then it is usually done by a member of the super user group.
Additional to the permissions are the View Access levels.

In general the View Access Level controls what a User Group may see and the permissions enable what a User Group may do.

If you explain a bit more what you are trying to do I will try to help
Best wishes
Colin