Part is not selectable when creating an assembly feature.
Solution
Open the part which could not be located, expand Design Body, right mouse button on the design body and select 'Activate Assembly Body'. Return to the assembly and try again.
Notes and References
Hardware/Software Configuration
Platform: INTEL
OS: windows
OS Version: n/a
Product: SOLID_EDGE
Application: ASSEMBLY
Version: V219MP8
Function: FEATURES
Ref: 001-9676197