TL;DR — I'd suggest: "declaração (or nota) de dívida" for pt-BR.
The OP is explicitly about pt-PT and is already answered in Lambie's post, but it seems to make more sense to address here too the translation to pt-BR than to have a separate question for it.
It's difficult to find a good translation, since the exact concept is not used in Brazilian Portuguese.
The most immediate translation could seem to be "nota promissória", but this is "promissory note", a more formal, negotiable instrument, as Lambie's answer explains. One could perhaps consider an IOU as an informal promissory note, what suggests the translation "nota promissória informal" — which is correct enough and understandable, but has the big disadvantage of never being used in practice (as Google's 6 hits attests).
Translation suggestions include "título de dívida", "vale", "recibo, and "adiantamento", none of which really works: "título de dívida" is "debt security", "debenture" or other financial asset; " "vale" is rather "coupon" or "voucher", thus a IOU of sorts, but only in specific, usually commercial contexts; "recibo is "receipt" and thus too general; and "adiantamento" is actually unrelated, meaning things such as "retainer" or "advance".
So we're left with using a description for a translation, such as, for pt-PT, "reconhecimento de dívida" ("acknowledgment of debt") or, for pt-BR, "declaração de dívida" ("statement or declaration of debt"). Notice that "reconhecimento de dívida" doesn't work so well in pt-BR because in Brazil this expression refers to a specific government issued document.