EDIT: Oh gees…there’s the problem:

'choices' => piklist_demo_get_states()

in line 86. My bad! Thanks so much for the help:)

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Leaving the following here in case anyone else makes the same mistake
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This is the error that comes up:

Fatal error: Uncaught Error: Call to undefined function piklist_demo_get_states() in /usr/www/users/beautinf/wp-content/plugins/symia-piklist/parts/meta-boxes/establishment-metabox.php:86 
Stack trace: 
#0 /usr/www/users/beautinf/wp-content/plugins/piklist/includes/class-piklist.php(523): include() 
#1 /usr/www/users/beautinf/wp-content/plugins/piklist/includes/class-piklist-meta.php(319): Piklist::render('/usr/www/users/...', Array) 
#2 /usr/www/users/beautinf/wp-admin/includes/template.php(1389): Piklist_Meta::meta_box(Object(WP_Post), Array) 
#3 /usr/www/users/beautinf/wp-admin/edit-form-advanced.php(686): do_meta_boxes(Object(WP_Screen), 'normal', Object(WP_Post)) 
#4 /usr/www/users/beautinf/wp-admin/post.php(206): require('/usr/www/users/...') 
#5 {main} thrown in /usr/www/users/beautinf/wp-content/plugins/symia-piklist/parts/meta-boxes/establishment-metabox.php on line 86

This is my /parts/meta-boxes/establishment-metabox.php file:

<?php
/*
Title: Establishment Metabox
Post Type: establishment
 */

piklist('field', array(
    'type' => 'select',
    'field' => 'establishment_type',
    'label' => 'Establishment Type',
    'choices' => array(
        'guesthouse' => 'Guesthouse',
        'villa' => 'Villa',
        'cottage' => 'Cottage',
        'apartment' => 'Apartment',
    ),
));

piklist('field', array(
    'type' => 'select',
    'field' => 'establishment_no_bedrooms',
    'label' => 'Number of Bedrooms',
    'choices' => array(
        '1_bedroom' => '1',
        '2_bedroom' => '2',
        '3_bedroom' => '3',
        '4_bedroom' => '4',
        '5_bedroom' => '5',
        '6_bedroom' => '6',
        '7_bedroom' => '7+',
    ),
));

piklist('field', array(
    'type' => 'textarea',
    'field' => 'rates',
    'label' => 'Rates',
    'add_more' => true,
    'attributes' => array(
      'rows' => 10,
      'cols' => 50,
      'class' => 'large-text',
    )
));

piklist('field', array(
    'type' => 'group'
    ,'field' => 'address_group' // removing this parameter saves all fields as separate meta
    ,'label' => __('Address (Grouped)', 'piklist-demo')
    ,'list' => false
    ,'description' => __('A grouped field with the field parameter set.', 'piklist-demo')
    ,'fields' => array(
      array(
        'type' => 'text'
        ,'field' => 'address_1'
        ,'label' => __('Street Address', 'piklist-demo')
        ,'required' => true
        ,'columns' => 12
        ,'attributes' => array(
          'placeholder' => 'Street Address'
        )
      )
      ,array(
        'type' => 'text'
        ,'field' => 'address_2'
        ,'label' => __('PO Box, Suite, etc.', 'piklist-demo')
        ,'columns' => 12
        ,'attributes' => array(
          'placeholder' => 'PO Box, Suite, etc.'
        )
      )
      ,array(
        'type' => 'text'
        ,'field' => 'city'
        ,'label' => __('City', 'piklist-demo')
        ,'columns' => 5
        ,'attributes' => array(
          'placeholder' => 'City'
        )
      )
      ,array(
        'type' => 'select'
        ,'field' => 'state'
        ,'label' => __('State', 'piklist-demo')
        ,'columns' => 4
        ,'choices' => piklist_demo_get_states()
      )
      ,array(
        'type' => 'text'
        ,'field' => 'postal_code'
        ,'label' => __('Postal Code', 'piklist-demo')
        ,'columns' => 3
        ,'attributes' => array(
          'placeholder' => 'Postal Code'
        )
      )
    )
  ));