Select the beam you want define lateral support.
Go to Design menu ---> Steel frame ---> View revise /overwrites
The you will be able to change the unbraced length. By default SAP assumes that all the beam is unbraced.
For example if your beam is braced in middle of their span , then put in Length Minor : 0.5
In order to be sure that every thing is allrigth, after running your model, check in " Check detail " that the allowable stress for the member is Ok.
Juan Fuentes