Why is the route not being built?
Most often there is no internet: the route is built by the Yandex routing service. You also need at least two points — otherwise the app reminds you: “Add at least two points”.
If there is a connection but the app says “Couldn’t build a route. Try other points or “Straight line”.”, there are no suitable roads between the points for the chosen mode: for example, a pedestrian cannot pass through a closed area, and a bus does not go there. Move the point, choose another mode or turn on “Straight line” — then the point travels directly, without roads.
A route that is already built travels without the internet, and a route from a GPX file needs no network at all: it follows the line from the file.