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Assembly/Part has references to parts that no longer exist in the Assembly.
There is no interactive object (like interpart expression, wave link, positioning constraints) in the assembly that has the reference and Part Cleanup does also not delete the wrong old reference.
Solution
In some of this cases the following Workaround helps:
1.Create a new empty dummy part with the same as the referenced part that can not be found.
2. Open the Assembly/Part with the problem and save it. Only the save operation deletes the wrong reference.
It is not necessary to save the dummy part file.
Notes and References
Hardware/Software Configuration
Platform: INTEL
OS: window
OS Version: 10_1809
Product: NX
Application: ASSEMBLIES
Function: UPDATESTRUCTURE
Ref: 002-7007349