You have a slash, astersisk, or other "illegal" character in a file or folder name.
Peter Spier - this is the file name - SIMPLYbostonMARCH Folder
Check the WHOLE path leading to the file(s).
The only other possibility is the file is locked.
I'm having the same issue.
There are no '/' in any of the file names (I've never used a / in a file name and never would).
I shortened long file names as it was suggested that this can cause a problem. Still won't work.
I tried copying and saving all links to a new folder as this was another suggestion I've encountered. In this case I got yet another error message stating 'Links could not be saved'
I have deleted and relinked everything. I've restarted InDesign. I've restarted my computer. I've resaved the file under a new name in a different place.
I have also run tests on other documents and none of them have issues packaging.
This is extremely frustrating.
Tedious solution but I resaved all the links manually to a new folder, relinked them and then packaged it and it went off without a hitch.