plugin functions don’t work in theme

Hi,
non of the plugin functions works in my themes function.
I’ve checked all of the plugin functions in this address:
https://dfactory.eu/docs/post-views-counter/developers-api/
but non of them works.
for example:

if (function_exists('pvc_get_post_views'))
    echo( "pvc defined" );
else
    echo( "pvc not defined" );

it always shows “Pvc not defined”
Please tell me how can I use plugin functions?