Does 上不去 mean "can't get on" or "won't get on"
In this context, the first one is correct - "I can't get onto the highway"


  我上不去


我(subject: I)

上(verb: get) 

不(potential particle: indicate 'unable') 

去(result/ direction particle: onto)


  上去= get onto; 上得去=（can get onto); 上不去= (can't get onto)


see comment from user6065 explaining potential complements (there's only two: 得 and 不)


  For the other, the phrase just meant that the subject would not get on


For this interpretation to be correct, the phrase had to be "我不上去"

In other context, "上去" could mean "go up" or "advance"

Example:

"上(二樓)去" = "go up to (second floor)"

"上(前面)去" = "advance to (the front)"

"上(高速公路)去" = "get onto (the highway)"