Blocks

Hello
I login in my site with administrator account but when i want to assign role for block in first page i got errorthat said:
Sorry, but you do not currently have permissions to do that (Assign roles to users)

and show this link:
http://docs.moodle.org/en/error/moodle/nopermissions


pleas help me
helpppppppp meeeeeeeeeeee