Metabox Callback Function for How to build Plugin series

gistfile1.php

<?php

/**
 * This is the entire callback function for the metabox. You can copy the whole thing or just the fields.
 */
 
function dwwp_meta_callback() {
	?>

	<div>
		<div class="meta-row">
			<div class="meta-th">
				<label for="job-id" class="dwwp-row-title">Job ID</label>
			</div>
			<div class="meta-td">
				<input type="text" name="job-id" id="job-id" value=""/>
			</div>
		</div>
		<div class="meta-row">
			<div class="meta-th">
				<label for="date_listed" class="dwwp-row-title">Date Listed</label>
			</div>
			<div class="meta-td">
				<input type="text" name="date_listed" id="date_listed" value=""/>
			</div>
		</div>
		<div class="meta-row">
			<div class="meta-th">
				<label for="application_deadline" class="dwwp-row-title">Application Deadline</label>
			</div>
			<div class="meta-td">
				<input type="text" name="application_deadline" id="application_deadline" value=""/>
			</div>
		</div>
		<div class="meta">
			<div class="meta-th">
				<span>Principle Duties</span>
			</div>
		</div>
		<div class="meta-editor"></div>
		<?php

		$content = get_post_meta( $post->ID, 'principle_duties', true );
		$editor = 'principle_duties';
		$settings = array(
			'textarea_rows' => 8,
			'media_buttons' => false,
		);

		wp_editor( $content, $editor, $settings);

	?>
		</div>
		<div class="meta-row">
	        <div class="meta-th">
	          <label for="minimum-requirements" class="wpdt-row-title"><?php _e( 'Minimum Requirements', 'hrm-textdomain' )?></label>
	        </div>
	        <div class="meta-td">
	          <textarea name="minimum-requirements" class ="hrm-textarea" id="minimum-requirements"><?php if ( isset ( $hrm_stored_meta['minimum-requirements'] ) ) echo esc_attr( $hrm_stored_meta['minimum-requirements'][0] ); ?></textarea>
	        </div>
	    </div>
	    <div class="meta-row">
        	<div class="meta-th">
	          <label for="preferred-requirements" class="wpdt-row-title"><?php _e( 'Preferred Requirements', 'hrm-textdomain' )?></label>
	        </div>
	        <div class="meta-td">
	          <textarea name="preferred-requirements" class ="hrm-textarea" id="preferred-requirements"><?php if ( isset ( $hrm_stored_meta['preferred-requirements'] ) ) echo esc_attr( $hrm_stored_meta['preferred-requirements'][0] ); ?></textarea>
	        </div>
	    </div>
	    <div class="meta-row">
	        <div class="meta-th">
	          <label for="relocation-assistance" class="prfx-row-title"><?php _e( 'Relocation Assistance', 'hrm-textdomain' )?></label>
	        </div>
	        <div class="meta-td">
	          <select name="relocation-assistance" id="relocation-assistance">
	              <option value="select-yes">Yes</option>';
	              <option value="select-no">No</option>';
	          </select>
	    </div> 
	</div>	
	<?php
}

$post->ID will only work if you use globally, so line 42 and 43 will read like:

global $post;
$content = get_post_meta( $post->ID, 'principle_duties', true );