can't / don't manage to

In the sentence:
‘I’ll write your report, but I can’t/ don’t manage to finish it before Friday.’
‘can’t’ is seen as a correct option and not ‘don’t manage to’. Could someone explain to me why?
Thank you a lot for your time!

Yes, ‘can’t manage to finish’ is the better (correct) option because it indicates his inability. In ‘don’t manage to finish’ you can find some sort of deliberate unconcerned attitude, and it is not suitable.