Call to undefined function get_userdata()

Hey Developers,
a customer of mine has a problem with one of my plugins and LearnPress.

We’re getting the following error when get_posts() is used anywhere in the dashboard.
It seems that LearnPress tries to hook into the parse_query hook and wants to use get_userdata() function. However this function is not existent at time of execution.

Uncaught Error: Call to undefined function get_userdata() in /app/public/wp-content/plugins/learnpress/inc/user/class-lp-user-factory.php:144

Maybe you can have a look at this. Thank you.