This is simply a permissions issue. The web server does not have the necessary rights to write to the mod/ directory.
How you fix this, depends on what operating system this is.
You can always install plugins the old fashioned way - instructions for Certificate are here: https://docs.moodle.org/32/en/Certificate_module#Installing
Personally, I prefer not to make my Moodle code files writeable as it slightly increases the risk of being hacked.