Error when activating the View Mode Page module
I don't know about the View Mode Page module specifically, but here's how you can find out yourself.


Check the issue queue for the module.  You can find a link to "all issues" on the module's project page (in this case, here).  In this case, there aren't many issues; if there were a lot, you can use the search function.  If searching, make sure to set the status to all issues, not just open issues, as if you're lucky your issue will already have been reported and fixed.
If step 1 doesn't solve the problem, create a fresh install of Drupal 7 on your local machine or another server and install the Views Mode Page module and its dependencies.  Make sure it works properly by itself when you have no other modules enabled.  (For a small module like View Mode Page which has less than a few hundred installs, this step is especially important; for an extremely common module like Views itself, you can assume that someone has already done this and it's your problem.)
If step 2 doesn't solve the problem, you'll need to start disabling the modules on your site one by one.  Of course, don't do this on your live site-- do it on a backup site.  Just guess which module might be causing the problem, disable it, and then clear the cache.  Then try to enable Views Mode Page.  If it doesn't work, choose another module to disable and repeat.


Yes, this is really painful and frustrating, but sometimes it is necessary if a search of the issue queues on drupal.org doesn't turn up anything.  If you do have to do this and find the source of the problem, please don't forget to create an issue on drupal.org and report what you found!