An operation won't schedule to the work unit in the routing, but will schedule to an alternative.

Operation Indicators

If the operation indicator is Red or Yellow the cause and solution will be different to those detailed here. Please see related articles.

Problem:

I am unable to schedule an operation to the work unit specified in the routing but the operation indicator is green. I can however schedule to another work unit. 
 

Cause(s) of the inability to schedule to the specified work unit:

• There is a global constraint set against the work unit in the routing
• There is a stock code constraint set against the work unit in the routing
• By planner access is in use and you therefore do not have access to the specified machine
• There is no available capacity on the work unit or the operation requires a greater number of resources than assigned to the work unit

To allow scheduling to another work unit one of the below must be in use:

• An alternative group has been set up
• Work unit mapping has been edited or removed

Solution(s) to enable scheduling to the specified Work unit:

• Global constraints can be used to either block scheduling to particular work units or control the time in which the operation will take vs the time defined in the operation.  A factor of 1 equals the routing time. Firstly check the resource type assigned to the work unit. Find this resource type in the constraints and verify the correct factors are set. Note: adding a zero in the wrong place can block scheduling all together. 
• Product constraints work in a similar way to global constraints but allow you to differentiate between stock codes. Further information on both types of constrain can be found by selecting the related articles.
• If you cannot see the required work unit in the production schedule you may not have access to it. Check user access settings and under the By planner tab you can control which planners have access to schedule each work unit. By default if no work units are checked this feature is not in use and there are no restrictions, however if the feature is in use you can check the work unit to enable visibility and scheduling.
• Check the required resources for the operation vs the resources on the work unit. If an operation requires two resources but the work unit only has one assigned scheduling will not be possible. Be careful increasing work unit resources to enable scheduling as this can inaccurately reflect capacity and result in overscheduling. Explore the float function to reduce consumed resources in line with available resources and extend the elapsed time of the operation.  If the work unit resource quantity is equal to or greater than the operation resource requirement it could also be that there is simply not enough time to complete the operation. This can either be before the end of the planning interval or before resources become reduced, either due to shift changes or other operations consuming the capacity. Extending the planning interval will allow you to determine if it is the length of the operation that is resulting in the inability to schedule it.  

Managing Alternatives & Mappings

• Alternative groups can be created to allow scheduling to work units other than those in the routing. If you are able to schedule to a work unit not in the routing this is one of 2 likely causes. 
• When work units are imported an automatic mapping is created. APS gives you the ability to amend these mappings. It is possible that the mapping has been removed or changed so that any operations requiring the work unit in question are scheduled elsewhere.

Please see related articles for further information.