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Hi @magick207:
You have used conf_info_bdg_affil
not only as a meta key, but also as a method, that is a class function that do not actually exists within the code base.
That said, please try with this code snippet instead:
<tr class="custom-field">
<th><?php _e( 'Custom field:', 'woocommerce-pdf-invoices-packing-slips' ); ?></th>
<td><?php $this->custom_field( 'conf_info_bdg_affil' ); ?></td>
</tr>