As an update, I needed to ensure Image/Edit all selected was chosen in the Capture menu. Thanks Catherine

I apologize if this is a simple user error but I'm working with a trial version of Helicon Focus with Capture One 21 and the Helicon plugin. I select 18 .nef photos, go to Image/Edit With/Stack with Helicon Focus. I continue with Edit With and the result is Helicon Focus opening with one .tif file ...