Mmm depends on what exactly isn´t letting you install. Sometimes when you download an app from the internet it tells you that it can´t be installed etc.
Usually what you do is enable an option that lets you install apps downloaded outside the mac app store. Go to:
System preferences --> security & privacy --> General tab (which is the first tab) and in the "allow apps downloaded from" select the "app store and identified developers" option.
Also after doing this, go to the app installer --> right click and in the dropdown menu select the "open" option, you should be prompted with another thing, hit ok and that should do.
Hope this helps. Have a good day