For anyone searching…
Although in most cases it’s probably better to include ACF in your plugin, you might have a situation where you know for a fact ACF is already present and you just want access to its functions.
In your plugin, you can run your kickoff function on acf/init like so:
add_action( 'acf/init', 'run_my_plugin' );
That will ensure your plugin runs after ACF has loaded, so you will have access to ACF’s functions.
Docs: https://www.advancedcustomfields.com/resources/acf-init/
