2you can’t solve this…
you can only: treat all Spain as 0% VAT or treat all Spain as 21% VAT
shopify is unwilling to fix this by creating another country (Canary Islands) for those provinces… their reply was like “you can do shopify plus to fix this”
same problem is for USA associated states (micronesia, palau, marshall islands, american samoa, guam, northern mariana islands, puerto rico, us virgin islands).. they should remove them from USA and create a separate country for each of them… this will fix also a lot of secondary things with integration with couriers like preview of shipping costs and waybill creation
the only way to fix this is pushing shopify to consider this solution…
on all others ecommerce software this is as i described