"Kupowałeś lody, gdy zgubiłeś portfel."

Translation: You were buying ice cream when you lost your wallet.

[Purpurlicht]

"You were buying ice cream when you lost the wallet"
-- You need the article "an" here. --

Whereas:
"You were buying ice cream when you lost your wallet."
Is given as a correct solution.

Posting this in case someone else makes the same mistake without seeing the hint. Just don't think it's a mistake.

[Toffolux]

"ice cream" or more specifically if referring to a particular treat "an ice cream" would both work

[Toffolux]

"you lost your wallet when (or while) you were buying ice cream" is equally as natural I think

[Jellei]

OK, added.