Learn Dutch: Zulke fabrieksfouten moeten wanneer dan ook kosteloos opgelost worden. - Such manufacturing defects must be solved free of charge whenever.

In Dutch, you can create "universal concessive pronouns" (words like whenever, whoever, whatever) by taking a question word and adding dan ook. Here, wanneer (when) + dan ook creates the meaning "no matter when" or "whenever". You can apply this exact same pattern to other question words, such as wat dan ook (whatever) or waar dan ook (wherever).

Why do we use Zulke here instead of Zo'n for "Such"?

Zo'n (short for zo een) is only used with singular nouns, like zo'n fabrieksfout (such a manufacturing defect). Because fabrieksfouten is plural, you have to use zulke.

Could I use gratis instead of kosteloos here?

Yes, structurally and grammatically you could, as both mean "free." However, kosteloos (literally "cost-less") sounds more formal, legal, or professional. It perfectly matches the business context of a company dealing with fabrieksfouten (manufacturing defects), whereas gratis is more common in everyday speech or retail advertising.

Why does the sentence end with opgelost worden instead of just oplossen?