The grammar is a bit wrong, but I guess it's to save money
“No step” is not a SENTENCE. It’s a LABEL.
It does not mean “DO NOT STEP HERE.” It means, “THIS IS NO STEP.”
“Step” here is a NOUN, not a verb. And that’s because, on wings of planes that DO have doors or canopies opening over the tops of the wings that require a person to walk on the wing top to get to the cabin entrance/exit, there IS a designated area, a “STEP,” sometimes called a “WING WALK,” which is indicated by markings to show where you SHOULD put your feet, like this:
https://www.quora.com/What-does-no-step-mean-when-written-on-aircraft