Yes, "Please have a seat" is a complete sentence. It contains a subject implied by the verb ("you") and conveys a clear request or command. The use of "please" makes it polite, enhancing its communicative effectiveness.
There is no direct object in this sentence.
no there is subject
Please complete your sentence.
Please complete your sentence.
Please put oscillation in a complete sentence.
Please complete your sentence.
'...not on the seat, is better, but is still not a complete sentence.
Please provide the sentence you would like me to complete.
Please restate your question as a complete sentence.
Please use a complete sentence.
Please phrase this in the form of a complete sentence.
Please make sure that your seat belt is securely fastened.