I am trying to Install  current version Of Invision forums  on localhost  buti am facing the  following issue.

An installation has already been activated for this license key. Your license key entitles you to one installation only. If you need to change the URL associated with your license, contact IPS technical support.

even   Invision  mention  we can add unlimited installation on  localhost

You need to include -TESTINSTALL.  As per the docs:

  Quote

Your self-hosted license allows you to have one live installation (your actual community) and one test installation. We also allow an unlimited number of installations on "localhost" domains so you can test locally without restriction.

If you are installing on a test install, enter your license key with -TESTINSTALL on the end. If you are installing on localhost (it must be "localhost" specifically, not "127.0.0.1" or any other alias), this will not count towards either of your installations, however, if you are installing on any other domain, it will count as your test installation.

 

@Adrian Pricina 

also, please note that you will not be able to install anything from the marketplace on a localhost test install.