Error message: The operation cannot be completed because the item "GraphPad Prism" is in use.

Prism Mac does not have an installation program. Instead, you simply drag the Prism application file (actually a package, if you know the distinction) into the Applications folder. Rarely this won't work, and you get a poorly worded message.

The only choice is to click OK, but of course things are not OK.

The first thing to do is get a cup of coffee, then try again in a few minutes. This can solve the issue. Tea may work too.

Then try these suggestions one by one:

• Relaunch Finder as explained here. 
• If you already have a version of Prism on your computer, either trash it or rename it, so there is no conflict.
• Launch Activity Monitor (/Applications/Utilities/Activity Monitor.app), make sure that "My Processes" is selected in the "Show" item on the toolbar, and then quit mdworker (part of Spotlight) and quicklookd32 (lets you preview files in Finder) processes. They will automatically start up when needed, so quitting those processes won't cause any problems. 
• Reboot the computer. 
• Google (or Bing) for the phrase, "operation cannot be completed in use". You'll find lots of hits, with lots of suggestions.