I can't run openssl rand -base64 32. I get this error :

openssl : The term 'openssl' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of 
the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:1
+ openssl rand -base64 32
+ ~~~~~~~
    + CategoryInfo          : ObjectNotFound: (openssl:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException