Problems such as yours are sometimes caused by files that should belong to you but are locked or have wrong permissions. This procedure will check for such files. It makes no changes and therefore will not, in itself, solve your problem.
First, empty the Trash.
Launch the Terminal application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go▹Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Terminal in the icon grid.
Triple-click anywhere in the line below to select it, then drag or copy it — do not type — into the Terminal window: