Translation:I lost my wallet at the store.
Wasureru has the meaning of "to leave something carelessly behind" though, as we were previously taught. So if you know you "lost" it in the store, wouldn't that be more appropriate than nakusu?
You could use "wasureru" in a similar circumstance to this question. However, "nakusu" means "to lose something" and "wasureru" means "to forget something" or as you pointed out "to leave something carelessly behind", so even if the meaning is similar, it's not the same. If the English says "lost", then I think it should be "nakushimashita".
If you know it's in the store... then i would argue that it is not lost - it is left carelessly behind. Lost implies (to me) that it is missing, but you do not know where where it is. The English sentence only works if you are physically in the store, and you think you lost your wallet in that store, but not sure where exactly.
I guess for me if it was 忘れました (wasuremashita), I would translate it as "I forgot my wallet at the store", as in I carelessly left it behind.
I'd say it's possible to know it's somewhere in the store, but not know WHERE exactly it is. Compare for example "I lost him in the forest." You know he's somewhere in the forest, but you don't know where.
Purse was marked wrong! This is correct - British English. Especially as it was a lady speaking.
Out of curiosity, do you not use the word "wallet" in British English?
Yep, wallets and purses both exist here, men use wallets, I'm sure some men use purses but you don't see it often here, women seem 50 50 split on wallet or purse usage, sometimes they use a wallet but refer to it as a purse to make things complicated, I've never asked my friend why she does that, but I'm assuming the answer is that she regards purse to be the femenine word for wallet.
My preference though is big ol' coin buckets. Can't live without 'em.
"I lost my purse in the shop." should be correct. さいふ means: purse; handbag; wallet
JLPT official exams could put spaces when everything is in hiragana for N5 and N4, but Duolingo (which targets approximately the same level) can't? It takes me forver to read these just because I constantly have to guess where the words are separated.
Japanese children's books do the same with the spacing, it does make reading easier (because I've also had to read the occasional children's book with no spaces, and that never goes well). When there's no spacing, you have to do your best to identify the particles, which helps divide the words.
財布をなくしました。Saifu o nakushimashita.
I lost my wallet. (I can't find it.)
財布がありません。 Saifu ga arimasen.
I don't have my wallet. (I forgot it at my house.)