If you cannot add a user to the system, it is more likely that it is due to your allocated role permissions. To assess whether you have permission to add a user, follow the following steps:

Step 1:

In the Administration tab, click on Role in the System group.

Step 2:
This will open the list of roles created on the site. Click on the role allocated to you.

Step 3:
On Permissions section, navigate to System Administration and check whether Manage Users is selected. This option authorizes the role specified to be able to add a new user to the system. If the tick-box is not selected, you do not have the permissions to add a user.

The same steps would apply of you would like to limit a user's access to certain aspects of the platform