You must consider that the command extends works in 2d, then on the xy work plane.
the only way to make it work vertically is to rotate the worktop.
simply rotate the ucts.
Alternatively, the fastest method, is to select the line, move the pointer to the end you need to stretch (without clicking) and wait for the menu to appear with the two elongation options. the first voice allows you to deform the line but risk losing the verticality while the second allows you to stretch the line while keeping its direction.
Now just select any point of the plan where you want your line to arrive (it must be a parallel xy worktop).
I don't have to extend the mancurrent, I was trying to create a niche next to it, in practice using the mancurrent and a low point as a reference, now I tried to rotate the ucs x;y, but I still don't find the extension points that intersect.
Are you using the line command? try using the perpendicular snap to the edge of the handler. if autocad should not respond, extract the edge from the solid