Cannot create domain list or delete field

[JWeiss_AP]

Hi community.

I've seen similar questions posted but none of the 'solutions' work for me. Our organisation hosts five core feature services on our Enterprise portal. Each of the core feature services contains three layers: a point, line and polygon layer. Each of these has a field called 'subtype' which has a List of Values (Domain), except for one layer for which a domain cannot be created, nor can the field be deleted. I'm hoping someone in the community has some potential solutions or reasons as to why only one particular layer and field has this issue. I cannot see any settings that would cause this to happen.