Try this
before value "," not .
Concat(Thisitem.'Assigned to',Value, ",")
not this one : Concat(ThisItem.'Assigned to'.Value, ",")
are you show that Assigned to value into Textinput or Dropdpown
Text input to use :- Concat(ThisItem.'Assigned to',Value, ",")
Dropdown to use :- ThisItem.'Assigned to'