Facebook Ads coupons are very rare these days (almost impossible to find)...
I can sell you 50$ Facebook coupons for new accounts (that means a recently created Fb ads account who never used a voucher before) for 25$ each.
So I wouldn't be able to use them for existing pages?
You can use it to promote existing pages, but you can't use it in old Facebook Ads accounts.
Basically you can't use the coupon in an existing Facebook account which is older than 30 days or you already used another coupon.