Which validation error message to use for user form?

Question

Which of these error message should I use for my add/edit user form.

(Although I am over thinking this)

Background

I have various forms around the system where assets can be assigned to specific users. When assigning assets they can leave the firstname and lastname blank meaning that the asset is assigned to specific department, If they enter an e-mail address they need to enter firstname and lastname.

Option 1

Must enter the firstname and lastname with an e-mail address

Option 2

Must enter the first and lastname with an e-mail address

Option 3

Must enter the firstname with an e-mail address
 Must enter the lastname with an e-mail address [seperate lines]

Option 4

Must enter the first and lastname

Answer 1

A plain language expert would be likely to recommend this:

Please enter first and last names.

Conciseness calls for reduction of words whenever possible; 'firstname' and 'lastname' is a classic example for this - both have 'name' so instead of AR + BR, prefer (A + B) * R. Since 'first' and 'last' are highly coupled, and 'please enter first' is meaningless, people shouldn't have problems understanding the sentence.

The sentence

Must enter X with an email address.

is unclear: Do I need to enter X and (together with) an email address? A better phrasing would be:

Please enter X when entering an email.

Regardless of anything, I would argue that the proposal itself suggests improvements are possible:

1. The logic of the form should be clear by design and not dependent on error messages.
2. As far as form validation goes, there is substantial evidence that the optimal user experience is field blur validation. Meaning validation (and confirmation) should take place as the user leaves the field. Feedback loops are the key reason for this. Consider the following redesign:

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Answer 2

Let the end-user first mention whether the asset is to be assigned to the User or Department. Based on this input, dynamically decide which other items are to be made available for input. This brings in clarity and clears all confusion.