nnet in caret. Bootstrapping or cross-validation?
There is a mistake in your train function.  It should be trControl
not trainControl.

By default the train function doesn't do any pre-processing
(preProcess = NULL)

Have trace = FALSE in the train function to stop printing the
trace to the screen.

You can find available model for caret at https://topepo.github.io/caret/available-models.html

Looks like nnet uses sigmoidal activation functions for the hidden layer; see here https://stats.stackexchange.com/questions/78252/whats-the-activation-function-used-in-the-nodes-of-hidden-layer-from-nnet-libra


Additionally, you might want to set linout parameter TRUE for regression and FALSE for classification.