Error message (Prism Mac): This operation can't be completed because the file "GraphPad Prism" is in use.

This can happen when installing GraphPad Prism Mac when the QuickLook plugin is indexing (and creating thumbnails) from recent Prism graphs. Wait a few minutes, and try again, and the installation should be fine.

If you are really inpatient, here is a way to avoid the wai time.  Run Activity Monitor, and find the "quicklookd32" process. Select that process,  click the "Quit Process" button on toolbar, and then click Quit. Now installing Prism should happen smoothly, without the error message.