Which phone specifically? The original Ace? Ace2? Ace3? Ace4?
What are you trying to download? What size are the file(s)?
At worst, you're running a 6-7 year old phone (Ace) Gingerbread (2.3.6), at best, it's JellyBean (4.2.2) or KitKat (4.4.2), a 3-4 year old phone (Ace4)
Prior to Android6.0, you cannot use the SD as "internal" storage, as that feature didn't come until much later than Gingerbread. What you need to be able to do on an older phone like that is download X file to the internal storage of the phone, then move it to the SD card.
If you're looking at the original Ace, it only had 512MB (1/2 GB) of storage, total. Available to the user is more like 180MB (<0.2GB), and that's before you install apps etc...