Using "could" for future events

Consider using "might not":

"I might not be present tomorrow."

"could not" is almost always found in sentences in the past tense with a completely different meaning to the verb:

"I could not complete the task."
as a substitute for:
"I was not able to complete the task."

In normal circumstances, people will say "I won't be able to be present tomorrow."

"I couldn't be present tomorrow" can only be interpreted as "I wouldn't be able to be present tomorrow(due to an implied condition).

Firstly, it's correct to say I couldn't be present tomorrow, but only if the pragmatic context makes sense for the listener. It will not fly as an example sentence in an English class because these tend to have "out of the blue" background information, and some modal expressions require background information to be felicitous.

When the modal verb is in its preterite form, it expresses remoteness rather than past time. The only different in meaning between I can't be present tomorrow and I couldn't be present tomorrow is that in the first case, the speaker's presence is not negotiable. In the second case, it could be that they are open to incentives, or that they are trying to dissuade the listener from formally asking them to be present.