That command is wrong because it is not a command, as it is more like a warning or fact. It is not being properly conjugated; to be written in a correct form it should be written as: NO CAIGAS.
<u>NO CAES</u> is written in the simple present form, the main verb <u>CAER</u> is only conjugated with the second-person singular <u>TÚ.</u>