TLC interprets formulas of the form
var = <expression> or var \in <expression>
as assignments in initial conditions (and similarly for actions). The formula
is not of this form, and also it does not properly define the value of rmState (e.g., it could be a function whose domain is a strict superset of RM) or even state that rmState is a function at all. TLC therefore does not interpret it as an assignment but as a standard state predicate to be evaluated. Because rmState does not yet have a value in the initial state that is being created, this leads to the error message that you have observed.
You could write
TCInit ==
/\ rmState \in [RM -> Status]
/\ \A r \in RM : rmState[r] = "working"
(where Status contains all possible statuses) but your definition of CInit is simpler and will be evaluated more efficiently by TLC.