"A mini truck is a vehicle that has an open backside to carry things from one place to other."
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But there has to be a tailgate (out of sight) to stop the load falling off the back.
That sentence is perfectly grammatical but makes no sense.
Perhaps:
The minitruck opens in the back so you can easily put things in it.:-|
It is neither correct (you have forgotten an indefinite article - again), nor a natural way to describe the van.
A flatbed truck is a truck with a bed that has no sides or tailgate.
That's remarkably similar to the photos Barb and Dave posted earlier. You could have told us then.
But ITS back is permanently open. It cannot be opened.