There are many reasons why you could get an 'Error' while downloading using FrostWire for Android.
1. Internet Connection
a) The most common issue is your internet connection. If for some reason FrostWire's access to the internet will be cut off, even for a second, during a download from Cloud Sources (YouTube, SoundCloud), then the download will go into an 'Error'. The download cannot recover, because the download links for each file in search results time out. If you think that's your issue, try using more stable Wi-Fi connection.
b) Cloud downloads (from YouTube or SoundCloud) will produce an error if the link to the file times out. When you search for a keyword and exit FrostWire then come back a few minutes later, the search results you've gotten before would time out and the download will result in an 'Error'.
c) If you get an 'Error' only on BitTorrent Downloads, your router needs some adjustment in settings or your internet provider is throttling or banning the BitTorrent traffic all together. We've even heard about mobile data providers cutting people's BitTorrent access on unlimited plans when their quota would go above 3-5GB.
What happens is that FrostWire fetches the .torrent file for you, but when it's ready to download the data via the BitTorrent network, it's access to the internet get's cut off.
In most cases, the file from BitTorrent network will Pause and wait for better network conditions to download, as it can recover without issues multiple times and continue downloading where it left off.
2. You just updated your Android OS
The solution depends on what Android version you've updated to - please see here for more info.
3. Huge file, slow device
Sometimes if a file is too big and your phone has trouble handling it. It happens on older phones and on phones with older Android operating systems. The amount of applications you have opened at any given time consumes your device's resources, so if you feel your device being too slow could be the case, restart your phone/tablet and open FrostWire ONLY to see how it will run.
4. Battery out during download
When your phone runs out of battery while downloading, your downloads will show an error next time you open FrostWire because certain processes were not shot down properly and transfers were not able to recover.
5. Trouble getting a .torrent file
On very rare occasion, there might be an issue downloading the .torrent file itself. You will see the download status go to "Waiting" and then "Error". In this situation, it's either the torrent file that's bad/fake/badly listed and there is nothing you can do about it other then downloading another file, or you can tap on the same torrent search result, and the 2nd time, the download it start correctly.
Always make sure you have the latest version of FrostWire, preferably the Plus version (it gets fixes first), not the Basic one from Google Play.
As your operating system updates and the sources FrostWire downloads from change their code, FrostWire has to keep updating as well - if you are running an old FrostWire, you will most certainly run into issues sooner or later.