Why can't I lock this dimension?
It's nothing special, just one hole and a apperent intersection.
I believe it is because it references a virtual intersection. Unless things changed, I think you need to place a point and constrain it to the two curves with "on element connect" and drive that with the dimension.
What color is this dimension? Is it red or blue? If it is red, then it is driving and such dimensions can be locked. If it is blue, then it is driven dimension and it can not be locked. Those dimensions are there just for reference, etc. They are not changing the model.
I guess, that dimension is blue. When you are dimensioning from the virtual intersection, the dimension is blue and can not be unlocked. If you want to dimension from this intersection, then you have to extent both lines to this intersection and then dimension from the endpoint of those lines.
Ken is right. You cannot modify this dimension if it connected to virtual point (UNL).
Ken has mentioned a solution and here is a video:
If you are in sync then this works fine. In trad sketch I've never seen this working without
placed point or other tricks...