couldn't?
 

I know this sentence is incorrect but can someone tell me why please. Thanks!

'Lets talk about this afternoon, if you couldn't find time for today, tomorrow will also be possible.

Why is it can't and not couldn't?

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.