Some Feature Layer geometry types i.e. multipoint are not supported by some of the style options i.e. heat map; however, there is no indication to the end user as to why the heat map option is not available for their 'point' layer. Its not always easy for an end user to deduce that their layer is a 'multipoint' geometry and thus not supported for heat map styling (currently they have to be able to view the layer's json which may not be readily available i.e. where the Services Directory has been disabled).
Note: This was previously reported as a bug https://support.esri.com/en/bugs/nimbus/QlVHLTAwMDEwMTQ2Nw== and the developers chose to simply remove the heat map option when a layer's geometry was not supported.
- updating the heat map style documentation to specify what geometries are supported (or not supported).
- specifying the layer's geometry in its item details page
- adding a warning to the web map's change style page that indicates the layer's geometry is not supported by the heat map option (esp. in the case of multipoint geometry layers).