Preview network fees

I'm trying to send bitcoin to an address but when i preview the transaction it won't let me and says;

"The selected account does not have enough funds to perform this transaction".

I have enough in my account to cover my payment, but I'm guessing the network fee is too high and I don't have enough. However, I can't see what the network fee is going to be, so have no idea how much to add to my account. Pretty sure I've never had this problem before, but I havent sent anything for a few months so not sure if something has changed.

Any ideas?