This is a powershell error message. In powershell you can set environment variables like this:
$Env:Foo = 'An example'
So your command should be look something like this
$Env:K6_BROWSER_ENABLED='true' ; k6 run .\pruebaNavegador.js
And I don’t know which k6 version do you use, but since v0.46.0 you don’t even need this environment variable to enable browser testing.