**What caused the problem**

I inspected your data. The problem is the geometry of your line. It's very weird. Vertices appear in a chaotic order: Start point (first vertex) of the line is somewhere in the middle, next few vertices follow the river, than it jumps to another part and continues there for a few vertices etc.

**How to solve the problem**

1. [`Explode your line`][1].

2. Select all features of the exploded line and [`Merge selected features`][2]. 

Than you can apply your label along the line where you want.



  [1]: https://docs.qgis.org/3.16/en/docs/user_manual/processing_algs/qgis/vectorgeometry.html?highlight=explode#explode-lines
  [2]: https://docs.qgis.org/3.16/en/docs/user_manual/working_with_vector/editing_geometry_attributes.html?highlight=merge#merge-selected-features