There was a post with this issue marked "solved" (therefore not open to new comments) by updating to the latest version, however I have the latest version and am experiencing the inability to delete pages, given the window "One or more of these pages are in use and could not be deleted". I now must drag them to the end of the document, and when done, split it & discard the file with the unwanted pages, but that is a lot of work with a lengthy document. If I knew what "in use" meant, I could rectify the situation.
Mac 10.9.5, Acrobat Pro 10.1.16
"In use" is not something that you did. Deep in the guts of Acrobat, when any tool is trying to do something with an object, it "acquires" that object, and once it's done, it "releases" the object again. When such an object is a page, and the page object is not released, then it will cause this "in use" error message you've seen when you try to delete it. There is very likely nothing you can do to fix this situation. Sometimes, the problem is caused by a 3rd party plug-in that does not correctly follow the rules you have to play by when acquiring/releasing things. If you have any 3rd party plug-ins installed, you could try to prevent them from loading by selecting to only load "certified" plug-ins on Acrobat's Preferences>General dialog:
If that's not it, then you are very likely out of luck.