Please check whether you have Photoshop CS6 set as Default Editor in Lightroom preferences.
Click on edit>preferences in Lightroom.
Click on the External Editing tab, Under additional external editor on the right hand side of application
Windows:-click on choose and navigate to C:\Program Files\Adobe\Adobe Photoshop Cs6 and choose the .exe file of photoshop.
Mac:-Select Photoshop.app from Applicationa>>Photoshop CC.
Also note that if you are trying to open a raw image that is only supported in Ps CC, then choosing PS Cs6 as default editor might not help.