Do you have to have certain pastlife karma to use a computer? So lets take all humans currently alive in the human world. They all have the karma to be reborn in this realm. 

Computers exist in our world, and in some places (Not all) public libraries provide them and access to them / the internet. 

Are there cases where karma would prevent someone from using a computer? Say even that one was right in front of them, would karmic forces prevent them from using it? Could karmic forces prevent them from touching it? 

Like say, if you tried to give to someone that had a lot of karma from stealing in a previous life. Would it be possible? Or would events always unfold that prevent you from giving in some way or another? 

What I am trying to ask is if karma can make events that seem possible conventionally (IE Hand someone a 20$ bill) be impossible in practice.
Karma is a complex dhamma that only Buddha saw it thru and thru..   However, Buddha taught us 6 things to know about the fundamentals of karma

For sure, Karma is not a fate.  In Angulimala Sutta, a Sutta about a misguided serial killer who would have committed matricide if Buddha didn't interfere with his killing mission.    

You can give $20 to someone but then you will never know if it was his good karma that let him run into you?  or his good karma that you have money to share.   Karma is a complex thing that is imponderable or incomprehensible, Acinteyya (Pali).