Receiving error: [debug] mod_headers.c(663): headers: ap_headers_output_filter()

once i incorporated this inside the htaccess file:

# 6 DAYS
<FilesMatch "\.(ico|pdf|flv|jpg|jpeg|png|gif|js|css|swf)$">
Header set Cache-Control "max-age=518400, public"

# 2 DAYS
<FilesMatch "\.(xml|txt)$">
Header set Cache-Control "max-age=172800, public, must-revalidate"

<FilesMatch "\.(html|htm)$">
Header set Cache-Control "max-age=7200, must-revalidate"

Any assistance is appreciated in regards to what I possibly could do in order to fix this?

Your configuration looks perfectly fine. You need to just change LogLevel from debug to warn (or whatever your preferred level is). Lower election me if I am wrong :)