incase you unistall the word fence plugin

just delete .usr.ini file its in hidden mode in your WordPress root directory.

Changing an account’s username or transferring an account to a server with a different home path will not update customizations made to the .user.ini files located in the wordpress install home folder. This will cause WordFence to access wordfence-waf.php at an incorrect location.

or

change the directory path that getting not found to correct

usr.ini file contains

; Wordfence WAF
auto_prepend_file = '/home/oldpath/public_html/wordfence-waf.php'
; END Wordfence WAF

my case in linux

is used  ls -a to show hidden file or .files

After cloning into a staging site or migrating your website to another host, the .user.ini still contains a link to the absolute path of the original web site’s wordfence-waf.php.