If this is the original code for a working plugin, that function would either be defined in the plugin itself or one of it's custom include files. So, it sounds like you don't have either the original complete plugin source code or you don't have the correct include files.
The closest reference I could find was zp_get_user_maxhealth() in
this post.
P.S. This better not be another ChatGPT thing . . .
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