无法制作隐藏/可见的错误消息

I am making a contact form, which includes name, email and a message which can then be submitted. If any of the three fields are null an error message is shown which I have working. The word "error" appears in all three field boxes. My issue is that I am trying to add a single error message which will be hidden and only appear if an error is made, this will say "Please fill in all fields before submitting" I am making a contact form, which includes name