Money and emotions.

Earning money is a skill. A skill that involves creating the conditions with intent that lead to consistent and sufficient income. 

Allocating or spending money is a different skill, one that requires us to prioritise investment over expenses and build a system that helps us do this in a sane manner. 

With both of them, the biggest challenge is emotional decision making which stops us from learning either of the skills or even appreciating that these are skills that can be learned.