Inflow or outflow flow direction lines

[Nana12]

Hello all, I'm trying to use the Enforce River Monotonicity geoproessing tool in 3D analyst and one of the inputs is to use Flow Direction(FD) as a line feature. I converted a regular FD raster to a polyline but it gives the following error;

Error 050195: In or out flow direction lines missing for polygon with OID = <value>

Solution : Create inflow and outflow present in the flow direction features

My question is, is there a way to create/identify inflow and outflow flow direction features?

The actual task i am trying out is hydro flattening of rivers in DEM and i am following the steps described here. https://www.esri.com/arcgis-blog/products/arcgis-pro/3d-gis/hydro-flattening-of-river-shorelines-in-... .

Thanks

[ClaytonCrawford]

It's likely the flow line end or start point(s) don't actually touch the river polygon boundary. I suggest using feature editing tools to snap the flow lines to the river polygon. There's also the possibility that the orientation of one or more of the flow lines is incorrect. Use arrowhead symbology on the flow lines to ensure they're headed in the right direction.