IMPORTANT: PLEASE DO ALWAYS INCLUDE THE VERSION OF THE TOOL YOU ARE USING!!! There are some known bugs in older versions, and without this info I can neither rule them out myself or point you to them, if there is a possibility you are encountering them.
In addition to what Simon said, please make sure that you don't have any prohibited characters ("/", "*", ":", "?", <", ">", "|") in the file path you selected to save your pack to. This should have been caught by the file browser for choosing the folder, but you never can be certain with them.
If none of this helps, then please try the following two things (one after the other):
1) Remove any of these prohibited characters from your pack name. I have dealt with these characters in rank and level names, but I am not sure about the pack title.
2) Remove any special sprites from your pack and then try to save it again. The usual sprites usually work fine, but I haven't tested it much with custom lemming sprites and the like. So there might still be some problem with them trying to be copied to a non-valid path.
I don't think it likely that either of them is the problem, but one can never know for certain...
If none of this helps, I'll try to release the next pack toolkit version tomorrow or so, and you try whether it works with the updated version. For the next version I replaced the file browser to select the folder to save to (among other things), so there are chances that the bug will magically disappear.
If not, then the next version will at least save the error message together with a stack trace to the file "ErrorLog.txt", which will help me immensly to determine the actual problem. Unfortunately I forgot to add this logging in the current version.
And yes, I still haven't gotten around to deal with these special folders like "Desktop", "Users", ...
