I have a book: 501 Latin Verbs: fully conjugated. In the conjugations for the verb sum, it leaves out the future imperatives. Are there no future imperatives for sum? So how would, "You must be noun/adjective by time or condition" translate?
There certainly is a future imperative of esse in Latin: see Allen and Greenough's New Latin Grammar:
IMPERATIVE PRESENT SING. 2. ĕs, be thou PLUR. 2. este, be ye FUTURE 2. estō, thou shalt be 2. estōte, ye shall be 3. estō, he shall be 3. suntō, they shall be
I have never met sunto in casual reading, though it often appears (per C.M. Weimer's comment) in legal contexts. Here is a classical example:
regio imperio duo sunto (Cic. de Legg. iii. 4)
There shall be two [consuls] for the royal empire.
In my reading, esto and estote appear more frequently. See, for example, an example from the Vulgate:
estote ergo vos perfecti sicut et Pater vester caelestis perfectus est (Matt 5:48)
Be ye therefore perfect as your heavenly Father is perfect.
The future imperatives are not the most common forms for necessity: in the example you give of "must," I would suggest translating with a passive periphrastic, e.g. legendum est tibi = you should read
The answer @brianpck gives is exactly right. However, the future imperative wouldn't work for your sentence, because the future imperative is used to describe long-lasting or permanent states, as in both his examples. If you were Minerva warning Paris against giving the Apple of Discord to the wrong goddess, you'd say (understanding that for esse the present subjunctive is usually substituted for the present imperative):
But if you were Ulysses leaving home knowing that this might be your last chance for a few decades to give Telemachus advice, you'd say
For your example I'd probably use either the present subjunctive plus the subjunctive in a fear clause:
Cras adsis ne moriaris!
Be there tomorrow, lest you die!
or a future more vivid condition:
Si cras non aderis, morieris.
If you're not there tomorrow, you will die.
Cato also uses the future imperative as how-to instructions in De Agricultura (wash your hands, salt the meat, etc.), but I've read neither any other Cato nor any other how-to instructions, so I don't know how universal that is.