On my last visit I was transiting so was never landside, and on the visit before that I had no reason to seek out a VAT refund so I'm afraid I don't know where this is in BCN but per AENA's (Spanish airport operator) website, for Terminal 1 it is located on Level 3 in the check-in area.
https://www.aena.es/en/josep-tarrade...at-refund.html.