There are two different fixes that can be applied depending on the version of SAM you are using:
1.) To enable Schedule Maintenance in Smarts SAM 7.2.x, do the following:
You can also enable this function from the command line:
dmctl -s INCHARGE-SA put ICS_InMaint::ICS-InMaint::isEnabled TRUE
2.) To enable Schedule Maintenance in Smarts SAM in versions 8.x and newer:
In Smarts SAM suite, your environment must have the Maintenance and Business Impact Manager (MBIM) service configured in order to use the Schedule Maintenance functionality (see Note statement). To enable the Schedule Maintenance function in Smarts SAM 8.x and newer versions, you must do the following:
From the Smarts SAM Global Administration Console, define and enable the MBIM domain manager.