I believe it indicates that you need to define a read-only tablespace for the referenced storage tier. When a lifecycle stage specifies the data to be read-only when it enters, ILMA will lookup the associated storage tier for the stage. Then, it must find a read-only tablespace on that tier. When you attempted to preview your simulation, the rule engine detected the missing tablespace.
To resolve this, go back to the storage tier setup and choose a tablespace for read-only data. Note, it can be a read-write tablespace as ILMA will pretend to clone the selected tablespace during simulation.