Faced the same problem today. Solved it by:
- Added any editable hosted feature layer with edition capabilities (in my case from an old Survey123) in the webmap
- Saved the webmap --> The "Use in Collector" checkbox appeared in the webmap settings and I could edit in the Fieldmap App
- Delete the newly added useless layer and saved the map again.
In my case, I suspect that the fact that I published layers only from ArcGIS Pro without Editing capabilities caused some weird problem after, even if I activated the edition later in AGOL.
There are some troobleshooting indication on that page:
Problem: The Use in ArcGIS Collector Option is Unavailable in a Web Map’s Settings (esri.com)
Cheers,
Léo