Answer:
The sentence you wrote is a negative imperative sentence, which means it gives a command or a request to not do something. To form a negative imperative sentence in English, you need to use the word "do" before the word "not" and then add the base form of the verb. The base form of a verb is the simplest form, without any prefixes or suffixes.
So, the correct way to complete your sentence is:
Visitors do not litter the beach and must carry their trash with them.
I hope this helps you.