According to a couple threads in this forum, the error is caused by left over files from a previous installation that cause problems with a new installation. Unfortunately, unless you are rooted, which gives you access to the system files, you can't get to the culprit files to delete them. If that's the case, you'll need to do a data factory reset, which deletes everything on your N7 and reverts it to its original, out of the box state. Obviously, it's best if you can somehow avoid a factory reset and the hassle of again customizing your N7 as you once had it.
Here are the threads from where I got this information. You might want to do further research to verify its accuracy.
skype wont install again
- Android Forums at AndroidCentral.com
Unknown Error code during application install: "-24" - Android Forums at AndroidCentral.com