It is trying to step the Ambulance agent because you’ve defined the model scheduler to be fastest which iterates over all agents in the model. Either you use a scheduler by type or you simply add another stepping function for Ambulance agent defining that it should do nothing for now, e.g. agent_step!(agent::Ambulance, model) = nothing. Since you probably want to do something with the ambulances later on anyway, I would suggest the latter approach.
On a side note, you don’t have to define the agent type with both @agent and mutable struct. These are redundant. Just use the @agent macro and that should do the trick.