Do you create PercEnv... values inside a reactive() call? (If you don't then they aren't reactive, and you don't need the brackets. The error message suggests they're not).
What version of R, Shiny and/or shinydashboard are you using? There are other arguments around colour, logos etc that you can pass to valueBox(), does using them change things?
Sorry but valueBoxOutput and the other features of shinydashboard, are only available when being used with a dashboardPage. The CSS is all defined at the page level. If you were sufficiently motivated, you could probably figure out the CSS styles you need and extract them from the shinydashboard package. (I'm guessing this, for a start?)
shinydashboard has a very strict structure, unlike Shiny. However, you can still do a LOT with it. Instead of the tabs in navbarPage, you can use the sidebar to get a multi-page app. See this for examples and a walk-through.