Although the error is cut off in zenkix's post, I'm guessing he got an UnauthorizedAccessException. Here are some possible reasons, in order of decreasing likelihood:
Running fCraft from a protected system directory (e.g. Windows or ProgramFiles folder) while using a non-administrative user account. In that case, fCraft's data should go into AppData, ProgramData, or some directory owned by your user account.
Accidentally running fCraft from a temporary folder. If you downloaded fCraft in a zip file, and tried to run it directly from the zip file, it'll end up being extracted and launched from a temporary folder. Instead, extract all files to a permanent location before running fCraft.
Launching two copies of fCraft. UnauthorizedAccessException can happen when two processes try to write to the same file at the same time. In that case, don't run two instances of fCraft from the same directory.
Launching a sandboxed, freshly-downloaded copy of fCraft. Open file properties, and see if there is a "Security" note indicating that the file came from the Internet, and an "Unblock" button next to it.
Trying to write to a read-only file. Make sure none of fCraft's files are marked as read-only (in files' properties).
Trying to write to a read-only drive. Make sure you're not running fCraft from a USB drive, CD/DVD, or a remote network share.
Trying to treat a directory as a file (e.g. saving a file with an exact name as an existing directory). Unlikely as it is, make sure there's no directory named "PlayerDB.txt" in fCraft's folders.
If all else fails, try to run fCraft as Administrator. UAC can sometimes get in the way even if you do everything right.