.htaccess not writeable – is set to 0644

I have WP Fastest Cache on several sites, this is the first time I’ve encountered this issue. I’ve just installed it and received the .htaccess is not writeable box directing me to manually modify it. When I follow the instructions, replace htaccess contents with the WPFC code, I get a 500 Server error. My htaccess permissions are set to 0644 per the WPFC info page.

Please help.

thank you, decli