Is rooting possible on an already encrypted device
In short: Yes, you can root an encrypted device, because "encryption" only encrypts /data and not /system. 

Are you sure you are using the correct settings in ODIN for your device and following all directions completely on whichever tutorial you are following? Please cross-reference your tutorial with the XDA Guide (highly recommended that you follow this guide; I can post the instructions here for permanent refernce upon request).