"Don't be sad!"
The verbs of quality ("be" verbs) cannot be used as a direct command. Someone cannot simply turn off and on their sadness. They must do something to add or remove their sadness. You must use an -'eghmoH construction to give a command for a verb of quality. It's not clear if the negative also requires -'eghmoH, but it would follow that it might and so in this course, that is the construction that we insist on.
It's not described in TKD, it's described in KGT (Klingon for the Galactic Traveler) on page 117.
Generally, when a verb describing a state of being (for example, tuj ["be hot"]) is used in the imperative form, the suffixes -'egh (reflexive suffix) and -moH ("cause") are used as well: **yItuj'eghmoH ("Heat yourself!" - that is, "Cause yourself to be hot!")...
The English "Don't be sad!" directly commands the listener(s) to not be in a certain state. Klingon (generally) does not allow this; you can only command someone to take an action, or (with -Qo') to not take it. Therefore stative commands must be formed as reflexive causatives. (Except when they're not. But this seems to be the normal way of doing it, at least.)
However, with a negative stative command, it seems to me that things become a bit more complicated, becuase the more complex predicate in the Klingon can be negated in different ways. If I'm right, "Don't be sad!" (with plural addressee) actually corresponds to two different Klingon commands: «pe'IQ'eghmoHQo'!» "Don't cause yourselves to be sad!" and «pe'IQbe''eghmoH!» "Cause yourselves to not be sad!".
Now, I thought of the above when I first did this question a while back, but I wasn't sure, at the time, whether «pe'IQbe''eghmoH!» was grammatical. Maybe any negation in a command had to be done with -Qo'. If you're wondering the same thing, a passage in the leaflet from qep'a' 2017 (quoted in a recent post on the tlhIngan-Hol mailing list) makes it clear that this is not the case. -Qo' is used for negative commands, when you're telling someone to refrain from an action. But when one commands someone to cause a negative, for instance, that's still a positive command and uses -be' and not -Qo'. (The example given is «HIleghbe'moH» "Cause me to not see!")
I don't know if this question accepts «pe'IQbe''eghmoH!» as an answer, but it should. (Assuming there's nothing else wrong with it that I've missed.) In fact, I think it might even be the better preferred answer, though «pe'IQ'eghmoHQo'!» should be accepted too; although "Don't be sad!" is a negative command in English, I think the Klingon positive command is closer to what is more often meant by it. It's not just about not actively making yourself sad. If you are sad, you're supposed to unsadden yourself.