Hello L.,
With !procedure or !function, we must use double or simple quote for arguments.
@startuml
!procedure note_over($target, $note)
note over $target: $note
!endprocedure
participant "Alice" as alice
note_over("alice", "some note")
@enduml
[See on PlantUML server]
Or explicitly mention '!unquoted':
@startuml
!unquoted procedure note_over($target, $note)
note over $target: $note
!endprocedure
participant "Alice" as alice
note_over(alice, "some note")
note_over(alice, other note)
@enduml
[See on PlantUML server]
If that can help,
Regards,
Th.