There's an issue with the updateConversions method CoordinateConversionViewModel | ArcGIS API for JavaScript 4.16 . Both in the documentation and in the types (typescript).
It's supposed to be updateConversions(location, conversions) , but it always comes back with an error saying the location is not a proper Point.
In the actual code it seems the properties are reversed:

it's actually looking for updateConversions(conversions, location)
in typescript the workaround I'm applying is:

Then it works fine.