Im not sure what the error is, but as nobody else has replied...
Try reuploading the functions.php file.
Have you made any edits to it for other plugins such as GoogleSEO etc?
What are you entering into your thread prefixes fields.
It doesn't seem that the OP can alter the prefixes crashing b4 it gets there ? I would guess the error lies in ./admin/modules/config/thread_prefixes.php
i just see the solution
just ACP >> tools and maintainent >> cache manager >> threadprefixes Rebuild Cache
CLEAR
thanks all for support