The problem what not only you have but all have, and it concerns not only the monorail station but also the train stations and also metro station, some objects are missing in the asset editor.
For a monorail/train/metro station to work you need the so-called station tracks, but strangely enough these are not available either in the asset editor or in the game itself, although the latter is understandable.
What distinguishes the station tracks from the normal tracks, quite simply, at the station tracks are left and right still pedestrian paths which are missing at the normal tracks.
The pedestrian paths are necessary because you can define here whether vehicles may stop here, in the Road-Editor under the Lane settings you will find the entry "Stop-Type", this means the placement of the stop, this can stand badly in the middle of the track.
You can avoid this if you go to the Road-Editor and create your own station tracks, add pedestrian paths and set the "Stop-Type" train/monorail/metro or whatever should stop there.
Why you should or must create these station tracks yourself when they are included in the game, Colossal-Order just has to fix the bug that they are available in the Asset-Editor.
The next problem is that most station tracks are longer than normal, a road and all other network structures are divided into segments and nodes.
By default, segments are a maximum of 12 grid blocks long, or 10, the 2 additional ones are each the nodes at the beginning and end of a segment.
Station tracks are longer than 12 grid blocks, e.g. 15 grid blocks in the Large Train Station, but they may not be longer than 22 grid blocks, i.e. 20 grid blocks for the Pure Segment, which corresponds to a maximum train length of 20 x 8 = 160 meters.
If such a station track is longer than 22 grid blocks, the error message "cannot find path" appears when creating a line.
To get such a track longer you can use the mod "Move-it".
There is another problem, but such station tracks would never spawn a train/monorail/metro, because you have to be able to set so-called spawn points, and this is not provided by the game and can only be enabled with a mod, mine is the "Advanced Asset Editor" mod.
You can write a bug report to Colossal Order.
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