Does 上不去 mean "can't get on" or "won't get on" In the context of getting onto an elevated highway, I've heard a misunderstanding between two native speakers about the phrase 我上不去.

For one of the speakers, the phrase meant an impossibility:  the subject had tried and failed to get on, or at least it was clearly impossible to get on the highway.

For the other, the phrase just meant that the subject would not get on, but didn't imply impossibility. The reason for not going might have been because it was impossible, or it might just have been that the subject didn't want to go.

Which is the correct meaning? Is there regional variation in this respect?
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)"