While opening the NX9 assembly file in NX11 it shows interpart expression message.
The missing Interpart Expression message customer is getting is not an error, it's a warning message users getting when they load an assembly/part where interpart expression are used and corresponding Part/assembly is not loaded in the current session. If you load those part/assembly then you won't get this warning message.
However, if you do not want this message to be appear, then kindly go to File > Utilities > Customer Defaults > Gateway > General > Expressions tab > switch OFF the option "Show Missing Interpart Expression Part Warnings" and Restart NX. Refer below snap for more details.
Notes and References
OS Version: 764SP1