Every pet entering Spain from the EU needs an electronic microchip (ISO-compliant), and MAPA's guidance confirms a legible tattoo applied before 3 July 2011 is also still accepted as an alternative, though a microchip is now standard practice. The chip has to be in place before, or at the same time as, the rabies vaccination — a vaccine given before the chip is implanted doesn't count toward the EU travel requirements.
The rabies vaccine can't be given until the animal is at least 12 weeks old, and it only becomes valid for travel 21 days after that first (primary) dose. MAPA's own page spells out the practical consequence directly: a pet under 15 weeks old simply cannot have cleared both the 12-week minimum and the 21-day wait, and is therefore not authorised entry into Spain at all — there's no shortcut or exception for a very young puppy or kitten.
Keep the microchip number, the vaccination date, and the pet passport entry consistent, and ask your vet to scan and confirm the chip number at every appointment. A mismatch between the chip number and the paperwork is one of the more common, entirely avoidable reasons pets get held up at a border or Traveler's Point of Entry check.