This function, call_user_func_array() is called by add_filter() (or add_action()) that was not declared properly. Scan the code for something like:

add_filter('something', 'smartwp_remove_wp_block_library_css')

to find the culprit. If you do find it and it wasn’t something you added/changed, you will have to troubleshoot to determine why this function was not declared/found.

The top menu is still there – you can see it very faintly at the very top of the page as you mouse over the area just above the warning message.

One last thing – it looks like you have display_errors enabled, which is not good practice for a public facing website.