error embed php files in template, after create plugin as instruction

[basketmen]

i want to embed php files in template

[Marco van Herwaarden]

As the error message already states: You are trying to include a file named "preview.php", which can not be found.`

[basketmen]

thanks for answering

[Marco van Herwaarden]

You are most likely somehwere using an include()/require() of functions.php, instead of a include_once() or require_once().

[basketmen]

i confuse why this is not works,

this is code for the plugin that i use

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