You will need to provide more info to get a good answer. Are you trying to flatten a surface or sheet metal part and what version are you using (SW2015, 2014,?)?
Sorry 2015 and im trying to flatten a surface, the sample part they give you to try out flatten.
I just tested this on my install of 2015 SP 0 and it seems to be working fine. If you are running network licenses, are you sure you have checked out a Premium license? I have no add-ins turned on and I don't believe there are any prerequisites other than Premium. If the problem persists, please contact your VAR for assistance.
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