[Help me] Worms W. M. D. Coupon

Most coupons don't stack with discounts by default. There are a few exceptions to the rule, but for the most part it's safe to assume they don't stack.

Weird, it looks like it should have disappeared from your inventory. When you add an item to your cart you have a coupon for, it gives you the option to apply that coupon at checkout. If you’re trying to check out but the store isn’t letting you use the coupon (even if you can’t combine the store let’s you choose which discount to apply) then it’s likely expired but still in your inventory due to a glitch or something.

Oh I had that happen before where it broke and just didn't leave my inventory. Trading it away made it fix itself

https://steamcommunity.com/games/327030/announcements/detail/1812041303117228487

vaild for 2 weeks , so it would have been march 11 meaning it's expired

Coupons you receive from completing badges don't stack with regular discounts. Coupons provided by devs/publisher do sometimes.
You'll have to wait until the discount ends to use the coupon.

It says "(Valid until 2019. 03. 11. 18:00:00)"
So I guess it expired but didn't disappear from your inventory for some reason.

Since it was still there I thought it was my mistake on the date format, thinking it was DD/MM/YYYY and it was MM/DD/YYYY. But I guess not.

So from your answers the coupon should have expired but it bugged. That makes sense.