'klarer ikke' means you could try, but you're not really able to do it very well at the moment. So you don't have the ability at the time.
'kan ikke' can have several meanings depending on the context
Not having enough knowledge or skill to try (I can't speak greek)
Simply being unable to perform the task (I can't fly)
In some cases: Not being allowed to (I can't stay up after bedtime)
"Jeg klarer ikke å danse nå" = "I'm not able to dance now"
"Jeg kan ikke danse" = "I can't dance"
I can't explain it in Norwegian but I hope English will do. Firstly, make it usually means survive so that changes the sense a lot. Also that implies doing something and failing. If you however ment we will not do it it doesn't say why you won't and that's the key. We will not manage to do it should pass. Klarer is not exactly about doing something or not but about whether, if you had to, for example, and wanted to you could do it. Precisely it means to be able to. I hope it helped :)