Cannot invoke initializer for type 'Double' with an argument list of type '(String?)'

amount?.count <= 0 here amount is optional. You have to make sure it not nil.

let amount:String? = amountTF.text
if let amountValue = amount, amountValue.count <= 0 {

}

amountValue.count <= 0 will only be called if amount is not nil.

Same issue for this let am = Double(amount). amount is optional.

if let amountValue = amount, let am = Double(amountValue) {
       // am  
}

Your string is optional because it had a '?", means it could be nil, means further methods would not work. You have to make sure that optional amount exists then use it:

WAY 1:

// If amount is not nil, you can use it inside this if block.

if let amount = amount as? String {

    let am = Double(amount)
}

WAY 2:

// If amount is nil, compiler won't go further from this point.

guard let amount = amount as? String else { return }

let am = Double(amount)