Cross - bad  With the release of ArcPro 3.2.1 new functionality was added by ESRI to all tool parameters that accept a table/layer. They now have a switch control to allow you to toggle on or off the use of existing selections. Changing this will have an adverse affect on RivEX and cause it to error.

Selection switch

The new switch control, ignore it!

Tick - good The solution is very simple, do not use it! The reason why this causes problems is that there is no programmatic way to determine it's state and it doesn't even appear if there is no selection on the layer. RivEX tools that take advantage of selections have a check box for you tick on if you want to use the selection.

Selection check box

Ignore what the switch is saying, do not touch it! If the RivEX tool uses a selection

it will always have a check box for you to tick on or off. Use that! In the screen shot

above both layers have selections, ignore the switches and what they are saying,

it's the site layer that can have a selection to be used.

You know this as it has a check box, the river network parameter does not.