 Re: couldn't?
can and could express different types of possibility.Think of can as meaning It is possible to XXX and could as meaning It is possible that X . For example :
You can get a driving licence when you're 18 ( = it's possible to get...) This is a fact.
A: Where's John? B: I'm not sure. He could be in Martina's office (= it's possible that he is in...) This is a hypothesis. The speaker doesn't know but is just imagining.
Your sentence converts to It's possible to... : If it's not possible for you to find time for it today... and therefore you need can
The "if" doesn't make it hypothetical. In essence what you are saying is : In the situation of you not having time today, then tomorrow is OK So we are assuming "no time" to be a fact for the purposes of the conversation.
This is a complex area and I've simplified considerably - apart from anything else because the technical terms used to describe the different types of possibility tend to be counter-intuitive and therefore confusing - so I've tried to avoid them. But the possible to/that guideline is a good rule of thumb.
My preferred version of your example would be : Let's talk about it this afternoon. Or if you can't find time for it today, then tomorrow. |