If Zalando gave you a discount code that doesn't work, check the rules of the code on Zalando's website or in the promotional email. Codes can only be used within a limited timeframe and typically cannot be combined with other vouchers. Some brands are excluded from these discounts.
Once you're sure your things meet the requirements, enter the code by hand at checkout instead of copying and pasting it. This will keep the formatting from going wrong. To get the best results, use the code on different browsers or devices (for example, switch from mobile to desktop). This is because Zalando's website sometimes has problems with code apps that are only present on certain platforms.
Talk to Zalando's customer service if the code still doesn't work. Click on "Help" on their website and use the live chat feature. Give the code and information about the sale, and if possible, ask for a manual application or a replacement code.
This is the information we have, but if you have more insights from your experience, please share in the comments.