Assembly features are constructed in an "ordered mode" fashion, so you need to define each step [plane, profile, extent] correctly, and also, what part[s] are to be included [normally the extent takes care of selection, but you can over-ride it] in the feature. These are normally trouble free to contruct, so is a little puzzling.
Can you record your screen, to include all steps, right up until it fails?.....that would help identify what might be causing the issue.
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