I put 'your wallet' which i realise literally is swoj portfel, but i also know in Polish people don't use 'swoj' much unless it's to clarify or correct, ie for emphasis and we put possession regularly, where in Polish it is not bothered with (because nothing is needed before a noun). But no matter, I guess I can see why it's marked wrong.
I understand the literal translation is "You put the wallet on the desk." In English, however, it could also be that we would leave off the "You" entirely in a case where we intend to give a clear instruction (where we were speaking directly to our intended audience). We'd simply say "Put the wallet on the desk". So should it be accepted without "You" as well?
"kłaść" (kładę, kładziesz, etc.) is for putting something 'on' something. On the table, on the shelf, on the bed.
"wkładać" (wkładam, wkładasz, etc.) is for putting something 'into' something. Into the wardrobe, into the fridge, into the box, etc.
Yes, it matters. Of course you'd be understood, but people would notice it's the wrong verb in fact.
"kładziesz" is a bit special, as both Accusative "na biurko" and Locative "na biurku" work, are natural, and mean basically the same - you can of course say that one focuses on the motion and one on the location where the wallet ends up, but that doesn't really change anything.