It might be possible system path has not been set yet. So, meanwhile try this solution ->
go to the directory where railsinstaller is installed copy path to railsinstaller->ruby2.2.0(or whatever you version is)->bin folder and add it to system path variable. and same for railsintaller->devkit->bin directory path and add that to system path. then close the terminal and reopen it go to your project directory check for the rails -v command if it shows you the version that mean everything is ok now you can now use rails command.
can you post a screeshot of your error?
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