swift 3 error : Argument labels ‘(_:)’ do not match any available overloads

You can do it this way:

public func currencyString(_ decimals: Int) -> String {

    let formatter = NumberFormatter()
    formatter.numberStyle = .currency
    formatter.maximumFractionDigits = decimals
    return formatter.string(from: NSNumber(value: decimals))!
}

Because if you check NSNumber you will get predefined init like:

public init(value: Int)