OrCAD capture differential pair greyed out problem

This is usually happens if you have enabled a Constraint Manager flow. (PCB - Constraint Manager). If you have then you would typically use CM to create differential pairs (select the two netnames in CM and right click - Create Differential Pair). If you have enable a CM flow and want to go back the only choice you have is to close Capture and in the directory where the design file is (filename.dsn) will be a filename.3sav (a backup of the dsn when you enabled CM) which can be renamed back to filename.dsn. You can then continue with non-cm enabled flow.