As a test you could try to program the agent on key 9.
If that fails, you could out one of the existing AGT appearances and then try to add your new one.
This will help determine if the problem has to do with Key 50, or perhaps a limit on AGT keys on a set.
Maybe this person is already assigned to a supervisor.
Thsi is from the NTP:
Agent lamp (Basic)
Up to 40 SL-1 telephone key/lamp pairs can be assigned to agent functions.
The M2216 telephone with two add-on modules supports up to 58 keys. The
key associated with each lamp is not used unless equipped with ACD
advanced features. The lamps show the state of individual agents in the
supervisor’s group. These lamp states are updated when the status of the agent
changes, independent of the supervisor’s mode of operation (Agent/
Supervisor/Make Set Busy). An agent can be assigned to only one supervisor
at a time.