The problem with RQuantlib is that it is not in the standard R package repository. So you must install it either directly as described in the manual, or via DRAT as described in the Stackoverflow link.

For this reason, dependencies such as Rcpp are also not automatically installed. This gives no error message, but calls to RQuantlib fail. So you must also manually install Rcpp. I believe that is the only dependency.

The message "could not find function AmericanOption" means that RQuantlib itself is not linked or not installed. AmericanOption is heavily used by contractVal, so this function would then also fail. If all is correctly installed, the response from R is:

Code:
> library('RQuantLib',quiet=TRUE)
> AmericanOption('put', strike=100, volatility=0.4, 100, 0.02, 0.03, 0.5, engine='CrankNicolson')
Concise summary of valuation for AmericanOption 
  value   delta   gamma    vega   theta     rho  divRho 
10.9173 -0.4358  0.0140      NA      NA      NA      NA


I've added some details to the manual description.