The PreferredNavData member of NavAgentProperties isn't properly initialized. It is using some constructor that doesn't seem to be valid. This is a regression from 4.19.2 (CL-4033788) as this same setup packages with no errors. This can be worked around by connecting a "Make Soft Class Path" to the "Preferred Nav Data" pin thereby initializing the property manually
This issue was reported and tested in 4.20.2 (CL-4302132). It was reproduced in Main 4.21 (CL-4313833)
Results: Packaging fails with error C2039: 'StaticStruct': is not a member of 'FSoftClassPath'
Expected: Packaging is successful
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-63364 in the post.