Make the Tax ID mandatory in the checkout page

The easiest way to make the tax/vat id mandatory in the checkout page is to change the is_required in the eav_attribute table

you can use this Query to change the value from 0 => 1

UPDATE `eav_attribute` SET `is_required` = '1' WHERE `eav_attribute`.`attribute_code` = 'vat_id';

or you can use this code when you want to use a custom extesnion

$installer->updateAttribute('customer_address', 'vat_id', 'is_required', true);

You will also need to display the Tax id in the frontend

Store->Configuration->Customers->Customers Configuration ->Name and Address Options

(Show Tax/Vat Number)

Store->Configuration->Customers->Customers Configuration->Create new Account Options

Show VAT Number on Storefront