A number of users have been reporting issues recently removing the protective seal on Windows 10 OEM System Builder product key stickers. Unlike retail Windows 10 licenses, which come on a business size card in the box; the OEM Windows 10 System Builder product key is located on the certificate of authenticity sticker on the outside or inside of the package. To prevent theft, a protective seal covers the product key. Similar to a pay as you go phone card, users can utilize a sharp instrument such as a knife, scissor or even a coin to remove protective seal. See below:

Credit: Microsoft

Sometimes, if too much pressure is applied or liquid comes in contact with the sticker, this can damage the product key characters. When this happens, your best course of action is to return it to the reseller for a replacement or refund; since it might be a defective product. Contacting Microsoft might be an option, although the software firm has no responsibility for damaged product keys; since production is handled by a third party source. If vendor is able to provide a replacement, ask them to remove the protective seal for you, so you are not liable for damage if it happens again.

If you are unable to get any assistance from the vendor, contact Microsoft support and they might be able to replace it for you.

http://support.microsoft.com/contactus/?ws=support#tab0

What if Microsoft can't help either?

Your options are limited, you might have to cut your losses, or try salvaging the key yourself. The first option you can try using is a magnifying glass to enlarge the characters. If you don't have a magnifying glass, you can use digital camera or your smart phone to take a picture then blow it up on your computer. Below is a list of characters used and not used in Microsoft product keys:

Characters and numbers used:
2346789BCDFGHJKMPQRTVWXY

Characters and numbers not used:
015AEILNOSUZ

Sometimes a 6 could be easily mistaken for a G or a B for an 8, so look carefully. If the product key sticker is smudged, you could try using a damp Q-Tip to gently scrub the smudge covering the letters.