The designer refuses to open a design (and sends you back to the Templates list) in two situations:

1. Every campaign that used this design has already moved past Created/Scheduled. More precisely: editing stays open as long as at least one campaign using the design is still Created or Scheduled; once none of the campaigns using it are in that state (they've all been Sent, are Sending/Outbox, or were Canceled/Errored), the design is frozen so the stats and preview keep matching what customers received. Clone the template (or choose it as a starting point in a new campaign) to keep working on it.
2. The design belongs to a flow that is currently ON. Turn the flow off first, then edit the email from the flow's Email block.

Separately, in the current version the Title field in Settings is always shown read-only once a design has been saved – not only for flow designs – so it can only be set when the design is first created.