(1) You could be trying to use an SLM12 module in an SL1600 unit as noted above.
(2) Try a different module. You could have a defective module.
(3) Modules must be installed from left to right with no open module positions in between modules.
(4) AC must be present before the controller will recognize newly added modules. Make sure the panel is tightly closed and try again.
(5) If none of the above works, try a Total Reset of the controller as explained in 7.1 of your manual.
(6) If none of the above works, replace the controller.